diff --git a/.chronus/changes/fix-versioning-sub-namespace-2024-4-2-22-46-51.md b/.chronus/changes/fix-versioning-sub-namespace-2024-4-2-22-46-51.md
new file mode 100644
index 0000000000..64456ec7fb
--- /dev/null
+++ b/.chronus/changes/fix-versioning-sub-namespace-2024-4-2-22-46-51.md
@@ -0,0 +1,8 @@
+---
+# Change versionKind to one of: internal, fix, dependencies, feature, deprecation, breaking
+changeKind: fix
+packages:
+  - "@typespec/versioning"
+---
+
+Fix crash when `@service` inside a versioned namespace
diff --git a/packages/versioning/src/versioning.ts b/packages/versioning/src/versioning.ts
index 9eefb0d044..68846d60a7 100644
--- a/packages/versioning/src/versioning.ts
+++ b/packages/versioning/src/versioning.ts
@@ -569,10 +569,10 @@ function resolveDependencyVersions(
  * @param program
  * @param rootNs Root namespace.
  */
-export function resolveVersions(program: Program, rootNs: Namespace): VersionResolution[] {
-  const versions = getVersion(program, rootNs);
+export function resolveVersions(program: Program, namespace: Namespace): VersionResolution[] {
+  const [rootNs, versions] = getVersions(program, namespace);
   const dependencies =
-    getVersionDependencies(program, rootNs) ??
+    (rootNs && getVersionDependencies(program, rootNs)) ??
     new Map<Namespace, Map<Version, Version> | Version>();
   if (!versions) {
     if (dependencies.size === 0) {
@@ -581,7 +581,7 @@ export function resolveVersions(program: Program, rootNs: Namespace): VersionRes
       const map = new Map();
       for (const [dependencyNs, version] of dependencies) {
         if (version instanceof Map) {
-          const rootNsName = getNamespaceFullName(rootNs);
+          const rootNsName = getNamespaceFullName(namespace);
           const dependencyNsName = getNamespaceFullName(dependencyNs);
           throw new Error(
             `Unexpected error: Namespace ${rootNsName} version dependency to ${dependencyNsName} should be a picked version.`
@@ -593,7 +593,7 @@ export function resolveVersions(program: Program, rootNs: Namespace): VersionRes
     }
   } else {
     return versions.getVersions().map((version) => {
-      const resolutions = resolveDependencyVersions(program, new Map([[rootNs, version]]));
+      const resolutions = resolveDependencyVersions(program, new Map([[rootNs!, version]]));
       return {
         rootVersion: version,
         versions: resolutions,
@@ -686,51 +686,62 @@ export function getVersionsForEnum(program: Program, en: Enum): [Namespace, Vers
 }
 
 export function getVersions(p: Program, t: Type): [Namespace, VersionMap] | [] {
-  if (versionCache.has(t)) {
-    return versionCache.get(t)!;
+  const existing = versionCache.get(t);
+  if (existing) {
+    return existing;
+  }
+
+  switch (t.kind) {
+    case "Namespace":
+      return resolveVersionsForNamespace(p, t);
+    case "Operation":
+    case "Interface":
+    case "Model":
+    case "Union":
+    case "Scalar":
+    case "Enum":
+      if (t.namespace) {
+        return cacheVersion(t, getVersions(p, t.namespace) || []);
+      } else if (t.kind === "Operation" && t.interface) {
+        return cacheVersion(t, getVersions(p, t.interface) || []);
+      } else {
+        return cacheVersion(t, []);
+      }
+    case "ModelProperty":
+      if (t.sourceProperty) {
+        return getVersions(p, t.sourceProperty);
+      } else if (t.model) {
+        return getVersions(p, t.model);
+      } else {
+        return cacheVersion(t, []);
+      }
+    case "EnumMember":
+      return cacheVersion(t, getVersions(p, t.enum) || []);
+    case "UnionVariant":
+      return cacheVersion(t, getVersions(p, t.union) || []);
+    default:
+      return cacheVersion(t, []);
   }
+}
 
-  if (t.kind === "Namespace") {
-    const nsVersion = getVersion(p, t);
+function resolveVersionsForNamespace(
+  program: Program,
+  namespace: Namespace
+): [Namespace, VersionMap] | [] {
+  const nsVersion = getVersion(program, namespace);
 
-    if (nsVersion !== undefined) {
-      return cacheVersion(t, [t, nsVersion]);
-    } else if (getUseDependencies(p, t) !== undefined) {
-      return cacheVersion(t, [t, undefined!]);
-    } else if (t.namespace) {
-      return cacheVersion(t, getVersions(p, t.namespace));
-    } else {
-      return cacheVersion(t, [t, undefined!]);
-    }
-  } else if (
-    t.kind === "Operation" ||
-    t.kind === "Interface" ||
-    t.kind === "Model" ||
-    t.kind === "Union" ||
-    t.kind === "Scalar" ||
-    t.kind === "Enum"
-  ) {
-    if (t.namespace) {
-      return cacheVersion(t, getVersions(p, t.namespace) || []);
-    } else if (t.kind === "Operation" && t.interface) {
-      return cacheVersion(t, getVersions(p, t.interface) || []);
-    } else {
-      return cacheVersion(t, []);
-    }
-  } else if (t.kind === "ModelProperty") {
-    if (t.sourceProperty) {
-      return getVersions(p, t.sourceProperty);
-    } else if (t.model) {
-      return getVersions(p, t.model);
-    } else {
-      return cacheVersion(t, []);
-    }
-  } else if (t.kind === "EnumMember") {
-    return cacheVersion(t, getVersions(p, t.enum) || []);
-  } else if (t.kind === "UnionVariant") {
-    return cacheVersion(t, getVersions(p, t.union) || []);
+  if (nsVersion !== undefined) {
+    return cacheVersion(namespace, [namespace, nsVersion]);
+  }
+
+  const parentNamespaceVersion =
+    namespace.namespace && getVersions(program, namespace.namespace)[1];
+  const hasDependencies = getUseDependencies(program, namespace);
+
+  if (parentNamespaceVersion || hasDependencies) {
+    return cacheVersion(namespace, [namespace, parentNamespaceVersion!]);
   } else {
-    return cacheVersion(t, []);
+    return cacheVersion(namespace, [namespace, undefined!]);
   }
 }
 
diff --git a/packages/versioning/test/versioned-dependencies.test.ts b/packages/versioning/test/versioned-dependencies.test.ts
index 54e6130a9c..652609c00d 100644
--- a/packages/versioning/test/versioned-dependencies.test.ts
+++ b/packages/versioning/test/versioned-dependencies.test.ts
@@ -790,6 +790,32 @@ describe("versioning: dependencies", () => {
     ok(v1.projectedTypes.get(MyService));
   });
 
+  // Regression test for https://github.com/microsoft/typespec/issues/3263
+  it("service is a nested namespace inside a versioned namespace", async () => {
+    const { MyService } = (await runner.compile(`
+      @versioned(Versions)
+      namespace My.Service {
+        enum Versions {
+          @useDependency(Lib.Versions.v1) v1
+        }
+      
+        @service
+        @test("MyService")
+        namespace Sub {
+          model Foo is Lib.Bar;
+        }
+      }
+      @versioned(Versions)
+      namespace Lib {
+        enum Versions { v1 }
+        model Bar {}
+      }
+    `)) as { MyService: Namespace };
+
+    const [v1] = runProjections(runner.program, MyService);
+    ok(v1.projectedTypes.get(MyService));
+  });
+
   // Test for https://github.com/microsoft/typespec/issues/786
   it("have a nested service namespace and libraries sharing common parent namespace", async () => {
     const { MyService } = (await runner.compile(`