diff --git a/app/controllers/application_controller.rb b/app/controllers/application_controller.rb
index ce001987c..5af81b87b 100644
--- a/app/controllers/application_controller.rb
+++ b/app/controllers/application_controller.rb
@@ -2,6 +2,15 @@ class ApplicationController < ActionController::Base
   include Auth
   include Ip
 
+  before_action -> {
+    return unless user_signed_in?
+
+    @organisation_list = current_user.all_organisations
+    if session[:current_user_organisation]
+      Current.user_organisation = session[:current_user_organisation]
+    end
+  }
+
   private
 
   def add_breadcrumb(name, path, options = {})
diff --git a/app/controllers/organisation_session_controller.rb b/app/controllers/organisation_session_controller.rb
new file mode 100644
index 000000000..a9e9d7c46
--- /dev/null
+++ b/app/controllers/organisation_session_controller.rb
@@ -0,0 +1,15 @@
+class OrganisationSessionController < ApplicationController
+  include Secured
+
+  def update
+    desired_organisation_id = params[:current_user_organisation]
+
+    if desired_organisation_id
+      if current_user.additional_organisations.pluck(:id).include?(desired_organisation_id) ||
+          current_user.primary_organisation.id == desired_organisation_id
+        session[:current_user_organisation] = desired_organisation_id
+      end
+    end
+    redirect_to current_user.service_owner? ? request.referer : root_path
+  end
+end
diff --git a/app/views/layouts/application.html.haml b/app/views/layouts/application.html.haml
index 2d8c1eaba..dc461efbf 100644
--- a/app/views/layouts/application.html.haml
+++ b/app/views/layouts/application.html.haml
@@ -62,6 +62,9 @@
             = t("header.feedback.html")
 
       = render 'layouts/messages'
+
+      = render "shared/organisation_switcher"
+
       = breadcrumb_tags
 
       = yield
diff --git a/app/views/shared/_navigation.html.haml b/app/views/shared/_navigation.html.haml
index ea4a7e0e6..fbbf2da8e 100644
--- a/app/views/shared/_navigation.html.haml
+++ b/app/views/shared/_navigation.html.haml
@@ -9,8 +9,8 @@
       %li{ class: navigation_item_class(reports_path) }
         = link_to t("page_title.report.index"), reports_path, class: "govuk-header__link"
 
-      %li{ class: navigation_item_class(organisation_activities_path(organisation_id: current_user.organisation_id)) }
-        = link_to t("page_title.activity.index"), organisation_activities_path(organisation_id: current_user.organisation_id), class: "govuk-header__link"
+      %li{ class: navigation_item_class(organisation_activities_path(organisation_id: current_user.organisation.id)) }
+        = link_to t("page_title.activity.index"), organisation_activities_path(organisation_id: current_user.organisation.id), class: "govuk-header__link"
 
       - if policy(:level_b).budget_upload?
         %li{ class: navigation_item_class(new_level_b_budgets_upload_path) }
@@ -20,8 +20,8 @@
         %li{ class: navigation_item_class(exports_path) }
           = link_to t("page_title.export.index"), exports_path, class: "govuk-header__link"
       - elsif policy([:export, current_user.organisation]).show?
-        %li{ class: navigation_item_class(exports_organisation_path(id: current_user.organisation_id)) }
-          = link_to t("page_title.export.index"), exports_organisation_path(id: current_user.organisation_id), class: "govuk-header__link"
+        %li{ class: navigation_item_class(exports_organisation_path(id: current_user.organisation.id)) }
+          = link_to t("page_title.export.index"), exports_organisation_path(id: current_user.organisation.id), class: "govuk-header__link"
 
       - if policy(Organisation).index?
         %li{ class: navigation_item_class(organisations_path) }
diff --git a/app/views/shared/_organisation_switcher.html.haml b/app/views/shared/_organisation_switcher.html.haml
new file mode 100644
index 000000000..ba5b5fa6c
--- /dev/null
+++ b/app/views/shared/_organisation_switcher.html.haml
@@ -0,0 +1,7 @@
+- if authenticated? && current_user.additional_organisations?
+  .govuk-grid-row
+    .govuk-grid-column-full{:class => "govuk-!-padding-top-4"}
+      =form_with url: organisation_session_path, method: :patch do |form|
+        =form.label t("organisation_switcher.label"), class: "govuk-label", for: "current_user_organisation"
+        =form.select :current_user_organisation, options_for_select(@organisation_list.pluck(:name, :id), Current.user_organisation || current_user.organisation.id), {}, class: "govuk-select"
+        =form.submit t("organisation_switcher.submit"), class: "govuk-button govuk-!-margin-bottom-1", "data-module": "govuk-button"
diff --git a/config/locales/views/organisation_switcher.en.yml b/config/locales/views/organisation_switcher.en.yml
new file mode 100644
index 000000000..e85b06435
--- /dev/null
+++ b/config/locales/views/organisation_switcher.en.yml
@@ -0,0 +1,5 @@
+---
+en:
+  organisation_switcher:
+    label: "Available organisations"
+    submit: "Set organisation"
\ No newline at end of file
diff --git a/config/routes.rb b/config/routes.rb
index 8ace6b8d5..7dd9a5b6b 100644
--- a/config/routes.rb
+++ b/config/routes.rb
@@ -32,6 +32,8 @@
   resources :users, except: [:index]
   resources :activities, only: [:index]
 
+  patch "/organisation_session" => "organisation_session#update"
+
   roles = %w[implementing_organisations partner_organisations matched_effort_providers external_income_providers]
   constraints role: /#{roles.join("|")}/ do
     get "organisations/(:role)", to: "organisations#index", defaults: {role: "partner_organisations"}, as: :organisations
diff --git a/spec/controllers/activities_controller_spec.rb b/spec/controllers/activities_controller_spec.rb
index 9cf536412..cd9b71ad7 100644
--- a/spec/controllers/activities_controller_spec.rb
+++ b/spec/controllers/activities_controller_spec.rb
@@ -184,6 +184,7 @@
     before do
       allow(controller).to receive(:current_user).and_return(user)
       allow(ActivityPolicy).to receive(:new).and_return(policy)
+      allow(user).to receive(:all_organisations).and_return([])
     end
 
     describe "#confirm_destroy" do
diff --git a/spec/controllers/application_controller_spec.rb b/spec/controllers/application_controller_spec.rb
index d81dbea5c..a8aeb03ed 100644
--- a/spec/controllers/application_controller_spec.rb
+++ b/spec/controllers/application_controller_spec.rb
@@ -1,5 +1,7 @@
 require "rails_helper"
 
+class DummyController < ApplicationController; end
+
 RSpec.describe ApplicationController, type: :controller do
   describe "#request_ip" do
     it "returns the anonymized v4 IP with the last octet zero padded" do
@@ -27,4 +29,49 @@
       end
     end
   end
+
+  describe "before_action" do
+    controller DummyController do
+      def custom_action
+        head 200
+      end
+    end
+
+    before(:each) do
+      routes.draw do
+        get "custom_action" => "dummy#custom_action"
+      end
+    end
+
+    context "user is not signed in" do
+      it "does not set Current.user_organisation" do
+        get "custom_action"
+
+        expect(Current.user_organisation).to be(nil)
+      end
+    end
+
+    context "user is signed in" do
+      let(:user) { create(:partner_organisation_user) }
+
+      before do
+        allow(controller).to receive(:current_user).and_return(user)
+      end
+
+      it "does not set Current.user_organisation if `current_user_organisation` is not in the session" do
+        get "custom_action"
+
+        expect(Current.user_organisation).to be(nil)
+      end
+
+      it "sets Current.user_organisation if `current_user_organisation` is in the session" do
+        id = SecureRandom.uuid
+        session[:current_user_organisation] = id
+
+        get "custom_action"
+
+        expect(Current.user_organisation).to be(id)
+      end
+    end
+  end
 end
diff --git a/spec/controllers/organisation_session_controller.rb b/spec/controllers/organisation_session_controller.rb
new file mode 100644
index 000000000..8044017b0
--- /dev/null
+++ b/spec/controllers/organisation_session_controller.rb
@@ -0,0 +1,47 @@
+require "rails_helper"
+
+RSpec.describe OrganisationSessionController do
+  let(:user) { create(:partner_organisation_user, organisation: organisation) }
+  let(:organisation) { create(:partner_organisation) }
+  let(:other_organisation) { create(:partner_organisation) }
+
+  before do
+    allow(controller).to receive(:current_user).and_return(user)
+
+    user.additional_organisations << [create(:partner_organisation), create(:partner_organisation)]
+  end
+
+  describe "#update" do
+    it "sets the session `current_user_organisation` to the user's primary organisation's ID" do
+      put :update, params: build_params(user.primary_organisation.id)
+
+      expect(session[:current_user_organisation]).to eq(user.primary_organisation.id)
+    end
+
+    it "sets the session `current_user_organisation` to an ID from the user's additional organisations" do
+      id = user.additional_organisations.pluck(:id).sample
+
+      put :update, params: build_params(id)
+
+      expect(session[:current_user_organisation]).to eq(id)
+    end
+
+    it "does not set the session `current_user_organisation` to a random ID" do
+      random_id = SecureRandom.uuid
+
+      put :update, params: build_params(random_id)
+
+      expect(session[:current_user_organisation]).not_to eq(random_id)
+    end
+
+    it "does not set the session `current_user_organisation` to an organisation ID not in the user's additional organisations" do
+      put :update, params: build_params(other_organisation.id)
+
+      expect(session[:current_user_organisation]).not_to eq(other_organisation.id)
+    end
+
+    def build_params(id)
+      {current_user_organisation: id}
+    end
+  end
+end
diff --git a/spec/features/users_can_edit_a_comment_spec.rb b/spec/features/users_can_edit_a_comment_spec.rb
index 6b52d236d..4465ab2c5 100644
--- a/spec/features/users_can_edit_a_comment_spec.rb
+++ b/spec/features/users_can_edit_a_comment_spec.rb
@@ -1,4 +1,6 @@
 RSpec.describe "Users can edit a comment" do
+  include HideFromBullet
+
   let(:beis_user) { create(:beis_user) }
   let(:partner_org_user) { create(:partner_organisation_user) }
 
@@ -77,7 +79,9 @@
           scenario "the user can edit comments on actuals belonging to the same organisation" do
             actual = create(:actual, :with_comment, report: project_activity_report, parent_activity: project_activity)
 
-            visit organisation_activity_comments_path(project_activity_comment.commentable.organisation, project_activity_comment.commentable)
+            skip_bullet do
+              visit organisation_activity_comments_path(project_activity_comment.commentable.organisation, project_activity_comment.commentable)
+            end
 
             expect(page).to have_link("Edit", href: edit_activity_actual_path(project_activity, actual))
           end
diff --git a/spec/features/users_can_switch_organisation_spec.rb b/spec/features/users_can_switch_organisation_spec.rb
new file mode 100644
index 000000000..aeea8a793
--- /dev/null
+++ b/spec/features/users_can_switch_organisation_spec.rb
@@ -0,0 +1,116 @@
+RSpec.feature "Users can switch organisation" do
+  context "when the user has additional organisations" do
+    let(:user_with_additional_organisations) do
+      org1 = create(:partner_organisation)
+      org2 = create(:partner_organisation)
+      user = create(:partner_organisation_user)
+      user.additional_organisations << [org1, org2]
+      user
+    end
+
+    before do
+      authenticate!(user: user_with_additional_organisations)
+      visit root_path
+    end
+    after { logout }
+
+    scenario "the organisation switcher dropdown is shown" do
+      expect(page).to have_content t("organisation_switcher.label")
+    end
+
+    scenario "the organisation switcher dropdown is populated correctly" do
+      user_with_additional_organisations.all_organisations.each do |org|
+        expect(page).to have_content org.name
+      end
+    end
+
+    scenario "the user can switch organisation" do
+      expect(page).to have_select("current_user_organisation", selected: user_with_additional_organisations.primary_organisation.name)
+
+      additional_org_name = user_with_additional_organisations.additional_organisations.first.name
+
+      select(additional_org_name, from: "current_user_organisation")
+      click_on t("organisation_switcher.submit")
+
+      expect(page).to have_select("current_user_organisation", selected: additional_org_name)
+    end
+
+    scenario "the nav links have the correct organisation ID when the user has switched organisation" do
+      additional_org = user_with_additional_organisations.additional_organisations.first
+
+      activities_href = organisation_activities_path(organisation_id: user_with_additional_organisations.primary_organisation.id)
+      exports_href = exports_organisation_path(id: user_with_additional_organisations.primary_organisation.id)
+
+      expect(page).to have_link(href: activities_href)
+      expect(page).to have_link(href: exports_href)
+
+      select(additional_org.name, from: "current_user_organisation")
+      click_on t("organisation_switcher.submit")
+
+      updated_activities_href = organisation_activities_path(organisation_id: additional_org.id)
+      updated_exports_href = exports_organisation_path(id: additional_org.id)
+
+      expect(page).to have_link(href: updated_activities_href)
+      expect(page).to have_link(href: updated_exports_href)
+    end
+
+    scenario "the Activities page shows the correct content when the user has switched organisation" do
+      additional_org = user_with_additional_organisations.additional_organisations.first
+
+      select(additional_org.name, from: "current_user_organisation")
+      click_on t("organisation_switcher.submit")
+
+      updated_activities_href = organisation_activities_path(organisation_id: additional_org.id)
+
+      visit(updated_activities_href)
+
+      expect(page).to have_content(t("page_title.activity.index"))
+    end
+
+    scenario "the Exports page shows the correct content when the user has switched organisation" do
+      additional_org = user_with_additional_organisations.additional_organisations.first
+
+      select(additional_org.name, from: "current_user_organisation")
+      click_on t("organisation_switcher.submit")
+
+      updated_exports_href = exports_organisation_path(id: additional_org.id)
+
+      visit(updated_exports_href)
+
+      expect(page).to have_content(t("page_title.export.organisation.show", name: additional_org.name))
+    end
+
+    scenario "the nav links have the primary organisation ID when the user has switched organisation and switched back" do
+      activities_href = organisation_activities_path(organisation_id: user_with_additional_organisations.primary_organisation.id)
+      exports_href = exports_organisation_path(id: user_with_additional_organisations.primary_organisation.id)
+
+      additional_org = user_with_additional_organisations.additional_organisations.first
+
+      select(additional_org.name, from: "current_user_organisation")
+      click_on t("organisation_switcher.submit")
+
+      updated_exports_href = exports_organisation_path(id: additional_org.id)
+
+      visit(updated_exports_href)
+
+      select(user_with_additional_organisations.primary_organisation.name, from: "current_user_organisation")
+      click_on t("organisation_switcher.submit")
+
+      expect(page).to have_link(href: activities_href)
+      expect(page).to have_link(href: exports_href)
+    end
+  end
+
+  context "when the user has no additional organisations" do
+    let(:user) { create(:partner_organisation_user) }
+
+    before { authenticate!(user:) }
+    after { logout }
+
+    scenario "the organisation switcher dropdown is not shown" do
+      visit root_path
+
+      expect(page).not_to have_content t("organisation_switcher.label")
+    end
+  end
+end
diff --git a/spec/rails_helper.rb b/spec/rails_helper.rb
index b83032d09..c423b8353 100644
--- a/spec/rails_helper.rb
+++ b/spec/rails_helper.rb
@@ -83,10 +83,12 @@
     ActionMailer::Base.delivery_method = :test
     ActionMailer::Base.perform_deliveries = true
     ActionMailer::Base.deliveries = []
+    ActiveSupport::CurrentAttributes.reset_all
   end
 
   config.after(:each) do |example|
     ActionMailer::Base.deliveries.clear
+    ActiveSupport::CurrentAttributes.reset_all
   end
 
   config.before(:each, type: :request) do
diff --git a/spec/views/shared/navigation_spec.rb b/spec/views/shared/navigation_spec.rb
index ea5894dd1..d851245ae 100644
--- a/spec/views/shared/navigation_spec.rb
+++ b/spec/views/shared/navigation_spec.rb
@@ -6,8 +6,10 @@
       allow(view).to receive(:policy) do |record|
         Pundit.policy(user, record)
       end
+
       allow(view).to receive(:current_user).and_return(user)
-      allow(user).to receive(:organisation_id).and_return(SecureRandom.uuid)
+      allow(user).to receive(:organisation).and_return(organisation)
+      allow(organisation).to receive(:id).and_return(SecureRandom.uuid)
     end
 
     render
@@ -15,25 +17,27 @@
 
   context "when the current user is a BEIS user" do
     let(:user) { build(:beis_user) }
+    let(:organisation) { build(:beis_organisation) }
 
     it "shows the link to the exports index" do
       expect(rendered).to have_link(t("page_title.export.index"), href: exports_path)
     end
 
     it "does not show the link to an organisation export page" do
-      expect(rendered).to_not have_link(t("page_title.export.index"), href: exports_organisation_path(id: user.organisation_id))
+      expect(rendered).to_not have_link(t("page_title.export.index"), href: exports_organisation_path(id: user.organisation.id))
     end
   end
 
   context "when the current user is a partner organisation user" do
     let(:user) { build(:partner_organisation_user) }
+    let(:organisation) { build(:partner_organisation) }
 
     it "does not show the link to the exports index" do
       expect(rendered).to_not have_link(t("page_title.export.index"), href: exports_path)
     end
 
     it "shows the link to the partner organisation's organisation export page" do
-      expect(rendered).to have_link(t("page_title.export.index"), href: exports_organisation_path(id: user.organisation_id))
+      expect(rendered).to have_link(t("page_title.export.index"), href: exports_organisation_path(id: user.organisation.id))
     end
   end
 end