diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5664621e6..823a006a0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,8 @@
 
 [Full changelog][unreleased]
 
+- Migration (and logic) to allow users to belong to multiple organisations
+
 ## Release 154 - 2024-12-05
 
 [Full changelog][154]
diff --git a/app/models/current.rb b/app/models/current.rb
new file mode 100644
index 000000000..2623ad992
--- /dev/null
+++ b/app/models/current.rb
@@ -0,0 +1,3 @@
+class Current < ActiveSupport::CurrentAttributes
+  attribute :user_organisation
+end
diff --git a/app/models/user.rb b/app/models/user.rb
index 90bac71db..0e73e2c58 100644
--- a/app/models/user.rb
+++ b/app/models/user.rb
@@ -3,6 +3,7 @@ class User < ApplicationRecord
     otp_secret_encryption_key: ENV["SECRET_KEY_BASE"]
 
   belongs_to :organisation
+  has_and_belongs_to_many :additional_organisations, class_name: "Organisation", join_table: "organisations_users"
   has_many :historical_events
   validates_presence_of :name, :email
   validates :email, with: :email_cannot_be_changed_after_create, on: :update
@@ -18,6 +19,25 @@ class User < ApplicationRecord
 
   delegate :service_owner?, :partner_organisation?, to: :organisation
 
+  def organisation
+    if Current.user_organisation
+      return Organisation.find(Current.user_organisation)
+    end
+    super
+  end
+
+  def primary_organisation
+    Organisation.find(organisation_id)
+  end
+
+  def all_organisations
+    Organisation.where(id: [organisation_id, additional_organisations.map(&:id)].flatten)
+  end
+
+  def additional_organisations?
+    additional_organisations.any?
+  end
+
   def active_for_authentication?
     active
   end
diff --git a/db/migrate/20241204220209_create_join_table_organisations_users.rb b/db/migrate/20241204220209_create_join_table_organisations_users.rb
new file mode 100644
index 000000000..be314c350
--- /dev/null
+++ b/db/migrate/20241204220209_create_join_table_organisations_users.rb
@@ -0,0 +1,8 @@
+class CreateJoinTableOrganisationsUsers < ActiveRecord::Migration[6.1]
+  def change
+    create_join_table :organisations, :users, column_options: {type: :uuid} do |t|
+      t.index [:organisation_id, :user_id]
+      t.index [:user_id, :organisation_id]
+    end
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 3355aa58a..596c0cb34 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2024_11_14_114012) do
+ActiveRecord::Schema.define(version: 2024_12_04_220209) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pgcrypto"
@@ -286,6 +286,13 @@
     t.index ["iati_reference"], name: "index_organisations_on_iati_reference", unique: true
   end
 
+  create_table "organisations_users", id: false, force: :cascade do |t|
+    t.uuid "organisation_id", null: false
+    t.uuid "user_id", null: false
+    t.index ["organisation_id", "user_id"], name: "index_organisations_users_on_organisation_id_and_user_id"
+    t.index ["user_id", "organisation_id"], name: "index_organisations_users_on_user_id_and_organisation_id"
+  end
+
   create_table "outgoing_transfers", id: :uuid, default: -> { "public.gen_random_uuid()" }, force: :cascade do |t|
     t.uuid "source_id", null: false
     t.uuid "destination_id", null: false
diff --git a/doc/architecture/decisions/0038-users-may-belong-to-multiple-organisations.md b/doc/architecture/decisions/0038-users-may-belong-to-multiple-organisations.md
new file mode 100644
index 000000000..23e80e490
--- /dev/null
+++ b/doc/architecture/decisions/0038-users-may-belong-to-multiple-organisations.md
@@ -0,0 +1,39 @@
+# 38. Users may belong to multiple organisations
+
+Date: 2024-12-05
+
+## Status
+
+Accepted
+
+## Context
+
+Some RODA users wish to have the ability to view reports, activities, etc from
+other organisations. Currently these users have to have their organisation
+changed for them so that they can do this, which requires manual intervention
+by an administrator.
+
+## Decision
+
+Much of the logic in RODA is predicated on the idea that one user belongs to
+one organisation; changing the relationship to one where a user has many
+organisations would require a very large engineering effort.
+
+We believe the easiest way to allow a RODA user belonging to one organisation
+to be able to view data belonging to other organisations is to maintain its
+existing relationship where it belongs to one organisation, but to add an
+additional relationship where a user has and belongs to many organisations,
+which we are calling "additional organisations". This way, all the existing
+logic and tests still work, but we can augment the functionality to allow a
+user to switch their current organisation to be a different one from an
+"additional organisations" list (the contents of which would be set by an
+administrator for a given user).
+
+## Consequences
+
+It will be easier for users who need to see data from other organisations to be
+able to do this without requiring manual intervention.
+
+The relationship between a user and its organisation is critical functionality
+and so we must tread carefully. Our proposed solution does not require any
+changes to the existing policy logic.
\ No newline at end of file
diff --git a/spec/models/user_spec.rb b/spec/models/user_spec.rb
index 598442d9d..00333f501 100644
--- a/spec/models/user_spec.rb
+++ b/spec/models/user_spec.rb
@@ -31,6 +31,70 @@
     # This also validates that the relationship is present
     it { is_expected.to belong_to(:organisation) }
     it { is_expected.to have_many(:historical_events) }
+    it { is_expected.to have_and_belong_to_many(:additional_organisations) }
+
+    it "has a primary organisation" do
+      user = create(:administrator)
+
+      expect(user.primary_organisation).to be_valid
+      expect(user.primary_organisation.id).to eq(user.organisation_id)
+    end
+
+    it "has additional organisations" do
+      org1 = create(:partner_organisation)
+      org2 = create(:partner_organisation)
+      org3 = create(:partner_organisation)
+
+      user = create(:administrator)
+      user.additional_organisations << [org1, org2, org3]
+
+      expect(user.additional_organisations.pluck(:id)).to include(org1.id)
+      expect(user.additional_organisations.count).to eq(3)
+    end
+
+    it "shows all organisations including the primary organisation" do
+      org = create(:partner_organisation)
+
+      user = create(:administrator)
+      user.additional_organisations << org
+
+      expect(user.all_organisations.size).to eq(2)
+      expect(user.all_organisations.pluck(:id)).to include(user.primary_organisation.id)
+      expect(user.all_organisations.pluck(:id)).to include(org.id)
+    end
+
+    it "determines whether there are additional organisations" do
+      org = create(:partner_organisation)
+
+      user = create(:administrator)
+      user.additional_organisations << org
+
+      expect(user.additional_organisations?).to eq(true)
+
+      user.additional_organisations = []
+      expect(user.additional_organisations?).to eq(false)
+    end
+
+    context "when the current organisation has been set" do
+      let(:current_organisation) do
+        create(:partner_organisation)
+      end
+
+      before do
+        Current.user_organisation = current_organisation.id
+      end
+
+      it "returns the current organisation instead of the primary organisation" do
+        user = create(:administrator)
+
+        expect(user.organisation).to eq(current_organisation)
+        expect(user.primary_organisation).not_to eq(current_organisation)
+      end
+
+      after do
+        Current.user_organisation = nil
+      end
+    end
   end
 
   describe "delegations" do