diff --git a/src/cargo/util/toml/mod.rs b/src/cargo/util/toml/mod.rs
index bc80097b705..5768825e938 100644
--- a/src/cargo/util/toml/mod.rs
+++ b/src/cargo/util/toml/mod.rs
@@ -2144,6 +2144,8 @@ fn to_dependency_source_id<P: ResolveToPath + Clone>(
                 .unwrap_or(GitReference::DefaultBranch);
             let loc = git.into_url()?;
 
+            warn_if_github_pull_request(&name_in_toml, &loc, manifest_ctx.warnings);
+
             if let Some(fragment) = loc.fragment() {
                 let msg = format!(
                     "URL fragment `#{fragment}` in git URL is ignored for dependency ({name_in_toml}). \
@@ -2182,6 +2184,30 @@ fn to_dependency_source_id<P: ResolveToPath + Clone>(
     }
 }
 
+/// Checks if the URL is a GitHub pull request URL.
+///
+/// If the URL is a GitHub pull request URL, an warning is emitted with a message that explains how
+/// to specify a specific git revision.
+///
+/// At some point in the future it might be worth considering making this a hard error, but for now
+/// it's just a warning. See <https://github.com/rust-lang/cargo/pull/15003#discussion_r1908005924>.
+fn warn_if_github_pull_request(name_in_toml: &str, url: &Url, warnings: &mut Vec<String>) {
+    if url.host_str() != Some("github.com") {
+        return;
+    }
+    let path_components = url.path().split('/').collect::<Vec<_>>();
+    if let ["", owner, repo, "pull", pr_number, ..] = path_components[..] {
+        let repo_url = format!("https://github.com/{owner}/{repo}.git");
+        let rev = format!("refs/pull/{pr_number}/head");
+        let warning = format!(
+            "dependency ({name_in_toml}) git url {url} is not a repository. \
+                The path looks like a pull request. Try replacing the dependency with: \
+                `git = \"{repo_url}\" rev = \"{rev}\"` in the dependency declaration.",
+        );
+        warnings.push(warning);
+    }
+}
+
 pub(crate) fn lookup_path_base<'a>(
     base: &PathBaseName,
     gctx: &GlobalContext,
diff --git a/tests/testsuite/bad_config.rs b/tests/testsuite/bad_config.rs
index 0fb00f8e8fa..dca1d77afa4 100644
--- a/tests/testsuite/bad_config.rs
+++ b/tests/testsuite/bad_config.rs
@@ -2151,6 +2151,44 @@ Caused by:
         .run();
 }
 
+#[cargo_test]
+fn github_pull_request_url() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.0.0"
+                edition = "2015"
+                authors = []
+
+                [dependencies.bar]
+                git = "https://github.com/foo/bar/pull/123"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("check -v")
+        .with_status(101)
+        .with_stderr_data(str![[r#"
+[WARNING] dependency (bar) git url https://github.com/foo/bar/pull/123 is not a repository. The path looks like a pull request. Try replacing the dependency with: `git = "https://github.com/foo/bar.git" rev = "refs/pull/123/head"` in the dependency declaration.
+[UPDATING] git repository `https://github.com/foo/bar/pull/123`
+...
+[ERROR] failed to get `bar` as a dependency of package `foo v0.0.0 ([ROOT]/foo)`
+
+Caused by:
+  failed to load source for dependency `bar`
+
+Caused by:
+  Unable to update https://github.com/foo/bar/pull/123
+...
+
+"#]])
+        .run();
+}
+
 #[cargo_test]
 fn fragment_in_git_url() {
     let p = project()
diff --git a/tests/testsuite/patch.rs b/tests/testsuite/patch.rs
index ff46b324091..0bc833f8b2d 100644
--- a/tests/testsuite/patch.rs
+++ b/tests/testsuite/patch.rs
@@ -357,6 +357,61 @@ fn patch_to_git() {
         .run();
 }
 
+#[cargo_test]
+fn patch_to_git_pull_request() {
+    Package::new("bar", "0.1.0").publish();
+
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.0.1"
+                edition = "2015"
+                authors = []
+
+                [dependencies]
+                bar = "0.1"
+
+                [patch.crates-io]
+                bar = { git = 'https://github.com/foo/bar/pull/123' }
+            "#,
+        )
+        .file(
+            "src/lib.rs",
+            "extern crate bar; pub fn foo() { bar::bar(); }",
+        )
+        .build();
+
+    p.cargo("check -v")
+        .with_status(101)
+        .with_stderr_data(str![[r#"
+[WARNING] dependency (bar) git url https://github.com/foo/bar/pull/123 is not a repository. The path looks like a pull request. Try replacing the dependency with: `git = "https://github.com/foo/bar.git" rev = "refs/pull/123/head"` in the dependency declaration.
+[UPDATING] git repository `https://github.com/foo/bar/pull/123`
+[WARNING] spurious network error (3 tries remaining): unexpected http status code: 404; class=Http (34)
+[WARNING] spurious network error (2 tries remaining): unexpected http status code: 404; class=Http (34)
+[WARNING] spurious network error (1 tries remaining): unexpected http status code: 404; class=Http (34)
+[ERROR] failed to load source for dependency `bar`
+
+Caused by:
+  Unable to update https://github.com/foo/bar/pull/123
+
+Caused by:
+  failed to clone into: [ROOT]/home/.cargo/git/db/123-[HASH]
+
+Caused by:
+  network failure seems to have happened
+  if a proxy or similar is necessary `net.git-fetch-with-cli` may help here
+  https://doc.rust-lang.org/cargo/reference/config.html#netgit-fetch-with-cli
+
+Caused by:
+  unexpected http status code: 404; class=Http (34)
+
+"#]])
+        .run();
+}
+
 #[cargo_test]
 fn unused() {
     Package::new("bar", "0.1.0").publish();