diff --git a/crates/uv-workspace/src/workspace.rs b/crates/uv-workspace/src/workspace.rs
index 5dc0c533fbc6c..068edcd61632b 100644
--- a/crates/uv-workspace/src/workspace.rs
+++ b/crates/uv-workspace/src/workspace.rs
@@ -60,6 +60,13 @@ impl WorkspaceCache {
         match result {
             Ok(workspace) => {
                 for package in workspace.packages.values() {
+                    // Historically, upward workspace discovery stopped at an intermediate
+                    // `pyproject.toml`, so don't map this member to the outer workspace in that
+                    // case.
+                    // See: <https://github.com/astral-sh/uv/issues/19916>
+                    if has_intermediate_pyproject(&workspace.install_path, &package.root) {
+                        continue;
+                    }
                     self.workspaces
                         .done(package.root.clone(), Ok(workspace.clone()));
                 }
@@ -99,6 +106,24 @@ impl WorkspaceCache {
     }
 }
 
+/// Returns `true` when a `pyproject.toml` sits between the member project directory and the
+/// workspace root.
+fn has_intermediate_pyproject(workspace_root: &Path, project_dir: &Path) -> bool {
+    if project_dir == workspace_root {
+        return false;
+    }
+
+    let Ok(_) = project_dir.strip_prefix(workspace_root) else {
+        return false;
+    };
+
+    project_dir
+        .ancestors()
+        .skip(1)
+        .take_while(|ancestor| *ancestor != workspace_root)
+        .any(|ancestor| ancestor.join("pyproject.toml").is_file())
+}
+
 #[derive(Debug, Clone)]
 pub struct WorkspaceError(Arc<WorkspaceErrorKind>);
 
diff --git a/crates/uv/tests/lock/lock.rs b/crates/uv/tests/lock/lock.rs
index 025133a473873..75f3863970e01 100644
--- a/crates/uv/tests/lock/lock.rs
+++ b/crates/uv/tests/lock/lock.rs
@@ -10228,6 +10228,119 @@ fn lock_no_workspace_source() -> Result<()> {
     Ok(())
 }
 
+/// Regression test for <https://github.com/astral-sh/uv/issues/19916>.
+///
+/// Lock a workspace with a member that also supports standalone installation via platform-specific
+/// path sources.
+#[test]
+fn lock_workspace_member_with_standalone_path_source() -> Result<()> {
+    let context = uv_test::test_context!("3.12");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(
+        r#"
+        [tool.uv.workspace]
+        members = ["root/app", "root/lib"]
+
+        [tool.uv.sources]
+        lib = { workspace = true }
+        "#,
+    )?;
+
+    let root = context.temp_dir.child("root");
+    root.child("pyproject.toml").write_str(
+        r#"
+        [project]
+        name = "root"
+        version = "0.1.0"
+
+        [tool.uv.sources]
+        lib = { path = "lib" }
+        "#,
+    )?;
+
+    root.child("app").child("pyproject.toml").write_str(
+        r#"
+        [project]
+        name = "app"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = ["lib"]
+
+        [tool.uv.sources]
+        lib = [
+            { path = "../lib", editable = true, marker = "sys_platform == 'darwin'" },
+            { path = "../lib", editable = true, marker = "sys_platform != 'darwin'" },
+        ]
+        "#,
+    )?;
+
+    root.child("lib").child("pyproject.toml").write_str(
+        r#"
+        [project]
+        name = "lib"
+        version = "0.1.0"
+        "#,
+    )?;
+
+    uv_snapshot!(context.filters(), context.lock(), @"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+
+    ----- stderr -----
+    Resolved 2 packages in [TIME]
+    ");
+
+    let lock = context.read("uv.lock");
+
+    insta::with_settings!({
+        filters => context.filters(),
+    }, {
+        assert_snapshot!(
+            lock, @r#"
+        version = 1
+        revision = 3
+        requires-python = ">=3.12"
+        resolution-markers = [
+            "sys_platform == 'darwin'",
+            "sys_platform != 'darwin'",
+        ]
+
+        [options]
+        exclude-newer = "2024-03-25T00:00:00Z"
+
+        [manifest]
+        members = [
+            "app",
+            "lib",
+        ]
+
+        [[package]]
+        name = "app"
+        version = "0.1.0"
+        source = { virtual = "root/app" }
+        dependencies = [
+            { name = "lib" },
+        ]
+
+        [package.metadata]
+        requires-dist = [
+            { name = "lib", marker = "sys_platform != 'darwin'", editable = "root/lib" },
+            { name = "lib", marker = "sys_platform == 'darwin'", editable = "root/lib" },
+        ]
+
+        [[package]]
+        name = "lib"
+        version = "0.1.0"
+        source = { editable = "root/lib" }
+        "#
+        );
+    });
+
+    Ok(())
+}
+
 /// Lock a workspace with a member that's a peer to the root.
 #[test]
 fn lock_peer_member() -> Result<()> {