diff --git a/packages/fresh/src/app.ts b/packages/fresh/src/app.ts
index fe1f740fb87..2031ee643e4 100644
--- a/packages/fresh/src/app.ts
+++ b/packages/fresh/src/app.ts
@@ -2,6 +2,7 @@ import { trace } from "@opentelemetry/api";
 
 import { DENO_DEPLOYMENT_ID } from "@fresh/build-id";
 import * as colors from "@std/fmt/colors";
+import { setBasePath } from "./runtime/shared.ts";
 import type { MaybeLazyMiddleware, Middleware } from "./middlewares/mod.ts";
 import { Context } from "./context.ts";
 import { mergePath, type Method, UrlPatternRouter } from "./router.ts";
@@ -186,6 +187,10 @@ export class App<State> {
   config: ResolvedFreshConfig;
 
   constructor(config: FreshConfig = {}) {
+    if (config.basePath !== undefined) {
+      this.#validateBasePath(config.basePath);
+    }
+
     this.config = {
       root: ".",
       basePath: config.basePath ?? "",
@@ -193,6 +198,38 @@ export class App<State> {
     };
   }
 
+  #validateBasePath(basePath: string): void {
+    if (basePath === "" || basePath === "/") {
+      return;
+    }
+
+    if (!basePath.startsWith("/")) {
+      throw new Error(
+        `Invalid basePath: "${basePath}". Must be empty, "/" or start with "/"`,
+      );
+    }
+
+    if (basePath.endsWith("/")) {
+      throw new Error(
+        `Invalid basePath: "${basePath}". Must not end with "/" except for root path`,
+      );
+    }
+
+    // Validate by round-tripping through URL — catches all invalid path chars
+    try {
+      const url = new URL(basePath, "https://localhost");
+      if (url.pathname !== basePath) {
+        throw new Error(
+          `Invalid basePath: "${basePath}". Contains characters that require encoding`,
+        );
+      }
+    } catch {
+      throw new Error(
+        `Invalid basePath: "${basePath}". Must be a valid URL path segment`,
+      );
+    }
+  }
+
   /**
    * Add one or more middlewares at the top or the specified path.
    */
@@ -336,10 +373,16 @@ export class App<State> {
       const cmd = app.#commands[i];
 
       if (cmd.type !== CommandType.App && cmd.type !== CommandType.NotFound) {
-        // Apply the inner app's basePath if it exists
         let effectivePattern = cmd.pattern;
         if (app.config.basePath) {
-          effectivePattern = mergePath(app.config.basePath, cmd.pattern, false);
+          // Avoid double basePath when mount path equals inner app's basePath
+          if (path !== app.config.basePath) {
+            effectivePattern = mergePath(
+              app.config.basePath,
+              cmd.pattern,
+              false,
+            );
+          }
         }
 
         const clone = {
@@ -383,6 +426,8 @@ export class App<State> {
       }
     }
 
+    setBasePath(this.config.basePath);
+
     const router = new UrlPatternRouter<Middleware<State>>();
 
     const { rootHandler } = applyCommands(
diff --git a/packages/fresh/src/app_test.tsx b/packages/fresh/src/app_test.tsx
index 67563b9f469..4f00ab49898 100644
--- a/packages/fresh/src/app_test.tsx
+++ b/packages/fresh/src/app_test.tsx
@@ -959,3 +959,24 @@ Deno.test("App - .mountApp() with both main and inner basePath", async () => {
   res = await server.get("/main/services/users");
   expect(res.status).toEqual(404);
 });
+
+Deno.test("App - .mountApp() avoids double basePath when mounting at same path as inner basePath", async () => {
+  const innerApp = new App({ basePath: "/ui" })
+    .get("/", () => new Response("ui home"))
+    .get("/dashboard", () => new Response("dashboard"));
+
+  const app = new App()
+    .get("/", () => new Response("root home"))
+    .mountApp("/ui", innerApp);
+
+  const server = new FakeServer(app.handler());
+
+  let res = await server.get("/");
+  expect(await res.text()).toEqual("root home");
+
+  res = await server.get("/ui");
+  expect(await res.text()).toEqual("ui home");
+
+  res = await server.get("/ui/dashboard");
+  expect(await res.text()).toEqual("dashboard");
+});
diff --git a/packages/fresh/src/runtime/server/preact_hooks.ts b/packages/fresh/src/runtime/server/preact_hooks.ts
index d33d61fee1a..ec6cb58d9ce 100644
--- a/packages/fresh/src/runtime/server/preact_hooks.ts
+++ b/packages/fresh/src/runtime/server/preact_hooks.ts
@@ -115,7 +115,7 @@ options[OptionsType.VNODE] = (vnode) => {
       setActiveUrl(vnode, RENDER_STATE.ctx.url.pathname);
     }
   }
-  assetHashingHook(vnode, BUILD_ID);
+  assetHashingHook(vnode, BUILD_ID, RENDER_STATE?.ctx.config.basePath);
 
   if (typeof vnode.type === "function") {
     if (vnode.type === Partial) {
@@ -301,8 +301,11 @@ options[OptionsType.DIFF] = (vnode) => {
 
               if (id.endsWith(".css")) {
                 items.push(
-                  // deno-lint-ignore no-explicit-any
-                  h("link", { rel: "stylesheet", href: asset(id) } as any),
+                  h(
+                    "link",
+                    // deno-lint-ignore no-explicit-any
+                    { rel: "stylesheet", href: asset(id) } as any,
+                  ),
                 );
               }
             }
@@ -470,14 +473,15 @@ function RemainingHead() {
     RENDER_STATE.islands.forEach((island) => {
       if (island.css.length > 0) {
         for (let i = 0; i < island.css.length; i++) {
-          const css = island.css[i];
-          items.push(h("link", { rel: "stylesheet", href: css }));
+          items.push(
+            h("link", { rel: "stylesheet", href: asset(island.css[i]) }),
+          );
         }
       }
     });
 
     RENDER_STATE.islandAssets.forEach((css) => {
-      items.push(h("link", { rel: "stylesheet", href: css }));
+      items.push(h("link", { rel: "stylesheet", href: asset(css) }));
     });
 
     if (items.length > 0) {
diff --git a/packages/fresh/src/runtime/shared.ts b/packages/fresh/src/runtime/shared.ts
index 6e05fed7d71..38e9aecc85d 100644
--- a/packages/fresh/src/runtime/shared.ts
+++ b/packages/fresh/src/runtime/shared.ts
@@ -2,6 +2,8 @@ import type { ComponentChildren, VNode } from "preact";
 import { BUILD_ID } from "@fresh/build-id";
 import { assetInternal, assetSrcSetInternal } from "./shared_internal.ts";
 
+let BASE_PATH = "";
+
 export { HttpError } from "../error.ts";
 
 /**
@@ -22,18 +24,23 @@ export { HttpError } from "../error.ts";
  */
 export const IS_BROWSER = typeof document !== "undefined";
 
+/** @internal Set the base path for asset URLs. Called once during app init. */
+export function setBasePath(basePath: string) {
+  BASE_PATH = basePath;
+}
+
 /**
  * Create a "locked" asset path. This differs from a plain path in that it is
  * specific to the current version of the application, and as such can be safely
  * served with a very long cache lifetime (1 year).
  */
 export function asset(path: string): string {
-  return assetInternal(path, BUILD_ID);
+  return assetInternal(path, BUILD_ID, BASE_PATH);
 }
 
 /** Apply the `asset` function to urls in a `srcset` attribute. */
 export function assetSrcSet(srcset: string): string {
-  return assetSrcSetInternal(srcset, BUILD_ID);
+  return assetSrcSetInternal(srcset, BUILD_ID, BASE_PATH);
 }
 
 export interface PartialProps {
diff --git a/packages/fresh/src/runtime/shared_internal.ts b/packages/fresh/src/runtime/shared_internal.ts
index d91d6e642ed..8d1cc880447 100644
--- a/packages/fresh/src/runtime/shared_internal.ts
+++ b/packages/fresh/src/runtime/shared_internal.ts
@@ -91,7 +91,11 @@ export const enum PartialMode {
  * specific to the current version of the application, and as such can be safely
  * served with a very long cache lifetime (1 year).
  */
-export function assetInternal(path: string, buildId: string): string {
+export function assetInternal(
+  path: string,
+  buildId: string,
+  basePath?: string,
+): string {
   if (!path.startsWith("/") || path.startsWith("//")) return path;
   try {
     const url = new URL(path, "https://freshassetcache.local");
@@ -102,7 +106,11 @@ export function assetInternal(path: string, buildId: string): string {
       return path;
     }
     url.searchParams.set(ASSET_CACHE_BUST_KEY, buildId);
-    return url.pathname + url.search + url.hash;
+    let finalPath = url.pathname + url.search + url.hash;
+
+    finalPath = applyBasePath(finalPath, basePath);
+
+    return finalPath;
   } catch (err) {
     // deno-lint-ignore no-console
     console.warn(
@@ -114,7 +122,11 @@ export function assetInternal(path: string, buildId: string): string {
 }
 
 /** Apply the `asset` function to urls in a `srcset` attribute. */
-export function assetSrcSetInternal(srcset: string, buildId: string): string {
+export function assetSrcSetInternal(
+  srcset: string,
+  buildId: string,
+  basePath?: string,
+): string {
   if (srcset.includes("(")) return srcset; // Bail if the srcset contains complicated syntax.
   const parts = srcset.split(",");
   const constructed = [];
@@ -127,7 +139,9 @@ export function assetSrcSetInternal(srcset: string, buildId: string): string {
     const leading = part.substring(0, leadingWhitespace);
     const url = trimmed.substring(0, urlEnd);
     const trailing = trimmed.substring(urlEnd);
-    constructed.push(leading + assetInternal(url, buildId) + trailing);
+    constructed.push(
+      leading + assetInternal(url, buildId, basePath) + trailing,
+    );
   }
   return constructed.join(",");
 }
@@ -139,15 +153,34 @@ export function assetHashingHook(
     ["data-fresh-disable-lock"]?: boolean;
   }>,
   buildId: string,
+  basePath?: string,
 ) {
   if (vnode.type === "img" || vnode.type === "source") {
     const { props } = vnode;
     if (props["data-fresh-disable-lock"]) return;
     if (typeof props.src === "string") {
-      props.src = assetInternal(props.src, buildId);
+      props.src = assetInternal(props.src, buildId, basePath);
     }
     if (typeof props.srcset === "string") {
-      props.srcset = assetSrcSetInternal(props.srcset, buildId);
+      props.srcset = assetSrcSetInternal(props.srcset, buildId, basePath);
     }
   }
 }
+
+/** Apply basePath to a given path string */
+export function applyBasePath(path: string, basePath?: string): string {
+  if (!basePath || basePath === "/") {
+    return path;
+  }
+
+  if (!path.startsWith("/") || path.startsWith("//")) {
+    return path;
+  }
+
+  // Avoid double-prefixing if the path already starts with basePath
+  if (path.startsWith(basePath + "/") || path === basePath) {
+    return path;
+  }
+
+  return basePath + path;
+}
diff --git a/packages/fresh/tests/basepath_test.tsx b/packages/fresh/tests/basepath_test.tsx
new file mode 100644
index 00000000000..280c4aff46d
--- /dev/null
+++ b/packages/fresh/tests/basepath_test.tsx
@@ -0,0 +1,66 @@
+import { expect } from "@std/expect";
+import { App } from "fresh";
+import { applyBasePath } from "../src/runtime/shared_internal.ts";
+
+Deno.test("basePath validation - rejects invalid paths", () => {
+  expect(() => new App({ basePath: "invalid" })).toThrow(
+    'Invalid basePath: "invalid". Must be empty, "/" or start with "/"',
+  );
+
+  expect(() => new App({ basePath: "/ui/" })).toThrow(
+    'Invalid basePath: "/ui/". Must not end with "/" except for root path',
+  );
+
+  expect(() => new App({ basePath: "/ui admin" })).toThrow(
+    'Invalid basePath: "/ui admin"',
+  );
+});
+
+Deno.test("basePath validation - accepts valid paths", () => {
+  expect(() => new App({ basePath: "" })).not.toThrow();
+  expect(() => new App({ basePath: "/" })).not.toThrow();
+  expect(() => new App({ basePath: "/ui" })).not.toThrow();
+  expect(() => new App({ basePath: "/api/v1" })).not.toThrow();
+  expect(() => new App({ basePath: "/ui-admin" })).not.toThrow();
+  expect(() => new App({ basePath: "/ui.test" })).not.toThrow();
+  expect(() => new App({ basePath: "/deep/nested/path" })).not.toThrow();
+});
+
+Deno.test("basePath validation - rejects relative paths", () => {
+  expect(() => new App({ basePath: "./" })).toThrow(
+    'Invalid basePath: "./". Must be empty, "/" or start with "/"',
+  );
+});
+Deno.test("applyBasePath - no basePath", () => {
+  expect(applyBasePath("/test", undefined)).toBe("/test");
+  expect(applyBasePath("/test", "")).toBe("/test");
+  expect(applyBasePath("/test", "/")).toBe("/test");
+});
+
+Deno.test("applyBasePath - relative paths not affected", () => {
+  expect(applyBasePath("test", "/ui")).toBe("test");
+  expect(applyBasePath("./test", "/ui")).toBe("./test");
+  expect(applyBasePath("../test", "/ui")).toBe("../test");
+});
+
+Deno.test("applyBasePath - absolute basePath", () => {
+  expect(applyBasePath("/test", "/ui")).toBe("/ui/test");
+  expect(applyBasePath("/api/users", "/ui")).toBe("/ui/api/users");
+  expect(applyBasePath("/", "/ui")).toBe("/ui/");
+});
+
+Deno.test("applyBasePath - complex paths", () => {
+  expect(applyBasePath("/api/v1/users", "/app")).toBe("/app/api/v1/users");
+  expect(applyBasePath("/assets/style.css", "/ui/admin")).toBe(
+    "/ui/admin/assets/style.css",
+  );
+});
+
+Deno.test("applyBasePath - non-absolute paths", () => {
+  expect(applyBasePath("http://example.com/test", "/ui")).toBe(
+    "http://example.com/test",
+  );
+  expect(applyBasePath("//cdn.example.com/test", "/ui")).toBe(
+    "//cdn.example.com/test",
+  );
+});
diff --git a/packages/plugin-vite/tests/build_test.ts b/packages/plugin-vite/tests/build_test.ts
index 0416c9f87f7..3cec7128526 100644
--- a/packages/plugin-vite/tests/build_test.ts
+++ b/packages/plugin-vite/tests/build_test.ts
@@ -726,3 +726,47 @@ Deno.test({
   sanitizeOps: false,
   sanitizeResources: false,
 });
+
+Deno.test({
+  name: "vite build - basePath asset links are correctly prefixed",
+  fn: async () => {
+    await using res = await buildVite(DEMO_DIR, { base: "/ui/" });
+
+    await launchProd(
+      { cwd: res.tmp },
+      async (address) => {
+        await withBrowser(async (page) => {
+          await page.goto(`${address}/ui/tests/css_modules`, {
+            waitUntil: "networkidle2",
+          });
+
+          // Test CSS links are prefixed correctly
+          const stylesheetHrefs = await page.evaluate(() => {
+            const links = Array.from(
+              document.querySelectorAll('link[rel="stylesheet"]'),
+            );
+            return links.map((link) => (link as HTMLLinkElement).href);
+          });
+
+          stylesheetHrefs.forEach((href) => {
+            expect(href).toMatch(/\/ui\/assets\/.*\.css/);
+          });
+
+          // Test image links are prefixed correctly
+          const imageSrcs = await page.evaluate(() => {
+            const images = Array.from(document.querySelectorAll("img"));
+            return images.map((img) => img.src).filter((src) =>
+              src.includes("/assets/")
+            );
+          });
+
+          imageSrcs.forEach((src) => {
+            expect(src).toMatch(/\/ui\/assets\//);
+          });
+        });
+      },
+    );
+  },
+  sanitizeOps: false,
+  sanitizeResources: false,
+});