diff --git a/.chronus/changes/fix-openapi3-union-names-2024-7-8-8-5-28.md b/.chronus/changes/fix-openapi3-union-names-2024-7-8-8-5-28.md
new file mode 100644
index 0000000000..1e12651d04
--- /dev/null
+++ b/.chronus/changes/fix-openapi3-union-names-2024-7-8-8-5-28.md
@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/openapi3"
+---
+
+Fix OpenAPI3 union names when declared within a namespace
diff --git a/packages/openapi3/src/schema-emitter.ts b/packages/openapi3/src/schema-emitter.ts
index 04b276afbd..36a2993220 100644
--- a/packages/openapi3/src/schema-emitter.ts
+++ b/packages/openapi3/src/schema-emitter.ts
@@ -475,7 +475,8 @@ export class OpenAPI3SchemaEmitter extends TypeEmitter<
 
   unionDeclaration(union: Union, name: string): EmitterOutput<object> {
     const schema = this.#unionSchema(union);
-    return this.#createDeclaration(union, name, schema);
+    const baseName = getOpenAPITypeName(this.emitter.getProgram(), union, this.#typeNameOptions());
+    return this.#createDeclaration(union, baseName, schema);
   }
 
   #unionSchema(union: Union): ObjectBuilder<OpenAPI3Schema> {
diff --git a/packages/openapi3/test/union-schema.test.ts b/packages/openapi3/test/union-schema.test.ts
index a30fec1637..3354a6ccf7 100644
--- a/packages/openapi3/test/union-schema.test.ts
+++ b/packages/openapi3/test/union-schema.test.ts
@@ -396,6 +396,63 @@ describe("openapi3: union type", () => {
     });
   });
 
+  it("handles unions defined in a namespace", async () => {
+    const res = await openApiFor(`
+      namespace Foo {
+        model A {
+          foo: string;
+        }
+      }
+
+      namespace Bar {
+        model A {
+          bar: string;
+        }
+      }
+
+      namespace Baz {
+        union A {
+          foo: Foo.A,
+          bar: Bar.A
+        }
+      }
+
+      @get
+      op getFoo(data: Baz.A): {};
+    `);
+
+    deepStrictEqual(res.components.schemas["Foo.A"], {
+      properties: {
+        foo: {
+          type: "string",
+        },
+      },
+      required: ["foo"],
+      type: "object",
+    });
+
+    deepStrictEqual(res.components.schemas["Bar.A"], {
+      properties: {
+        bar: {
+          type: "string",
+        },
+      },
+      required: ["bar"],
+      type: "object",
+    });
+
+    deepStrictEqual(res.components.schemas["Baz.A"], {
+      anyOf: [
+        {
+          $ref: "#/components/schemas/Foo.A",
+        },
+        {
+          $ref: "#/components/schemas/Bar.A",
+        },
+      ],
+    });
+  });
+
   it("throws diagnostics for empty enum definitions", async () => {
     const diagnostics = await diagnoseOpenApiFor(`union Pet {}`);