diff --git a/.changeset/grumpy-turkeys-flow.md b/.changeset/grumpy-turkeys-flow.md
new file mode 100644
index 00000000..1e4c8812
--- /dev/null
+++ b/.changeset/grumpy-turkeys-flow.md
@@ -0,0 +1,5 @@
+---
+'@hono/swagger-ui': patch
+---
+
+Add explicit return types
diff --git a/packages/swagger-ui/src/index.ts b/packages/swagger-ui/src/index.ts
index a5dc37a3..e5e0ee1f 100644
--- a/packages/swagger-ui/src/index.ts
+++ b/packages/swagger-ui/src/index.ts
@@ -40,7 +40,7 @@ type OriginalSwaggerUIOptions = {
 
 type SwaggerUIOptions = OriginalSwaggerUIOptions & DistSwaggerUIOptions
 
-const SwaggerUI = (options: SwaggerUIOptions) => {
+const SwaggerUI = (options: SwaggerUIOptions): string => {
   const asset = remoteAssets({ version: options?.version })
   delete options.version
 
diff --git a/packages/swagger-ui/src/swagger/renderer.ts b/packages/swagger-ui/src/swagger/renderer.ts
index 6a2b012d..06b61ed6 100644
--- a/packages/swagger-ui/src/swagger/renderer.ts
+++ b/packages/swagger-ui/src/swagger/renderer.ts
@@ -123,7 +123,7 @@ const RENDER_TYPE_MAP = {
   (typeof RENDER_TYPE)[keyof typeof RENDER_TYPE]
 >
 
-export const renderSwaggerUIOptions = (options: DistSwaggerUIOptions) => {
+export const renderSwaggerUIOptions = (options: DistSwaggerUIOptions): string => {
   const optionsStrings = Object.entries(options)
     .map(([k, v]) => {
       const key = k as keyof typeof RENDER_TYPE_MAP
diff --git a/packages/swagger-ui/tsconfig.build.json b/packages/swagger-ui/tsconfig.build.json
index ccc2f65a..a82a1266 100644
--- a/packages/swagger-ui/tsconfig.build.json
+++ b/packages/swagger-ui/tsconfig.build.json
@@ -4,7 +4,8 @@
     "rootDir": "src",
     "outDir": "dist",
     "tsBuildInfoFile": "dist/tsconfig.build.tsbuildinfo",
-    "emitDeclarationOnly": false
+    "emitDeclarationOnly": false,
+    "isolatedDeclarations": true
   },
   "include": ["src/**/*.ts"],
   "exclude": ["**/*.test.ts"],