diff --git a/packages/api/src/keymanager/routes.ts b/packages/api/src/keymanager/routes.ts
index ff4148fa7af8..7aa588da778f 100644
--- a/packages/api/src/keymanager/routes.ts
+++ b/packages/api/src/keymanager/routes.ts
@@ -7,7 +7,7 @@ import {
   MAX_BUILDER_URL_SIZE,
 } from "@lodestar/params";
 import {Epoch, phase0, ssz, stringType} from "@lodestar/types";
-import {isValidAsciiHttpUrl} from "@lodestar/utils";
+import {isValidAsciiHttpUrl, toAsciiHttpUrl} from "@lodestar/utils";
 import {
   EmptyArgs,
   EmptyMeta,
@@ -194,7 +194,11 @@ export function builderConfigDataFromJson(json: unknown): BuilderConfigData {
         throw Error(`builders[${i}].url must not exceed ${MAX_BUILDER_URL_SIZE} bytes`);
       }
       if (!isValidAsciiHttpUrl(url)) {
-        throw Error(`builders[${i}].url must be a valid HTTP or HTTPS URL using only ASCII characters`);
+        const asciiUrl = toAsciiHttpUrl(url);
+        throw Error(
+          `builders[${i}].url must be a valid HTTP or HTTPS URL using only ASCII characters` +
+            (asciiUrl !== null ? `, use ${asciiUrl} instead` : "")
+        );
       }
       if (auth_data !== undefined && (typeof auth_data !== "string" || !BUILDER_AUTH_DATA_PATTERN.test(auth_data))) {
         throw Error(
diff --git a/packages/api/test/unit/keymanager/builderConfig.test.ts b/packages/api/test/unit/keymanager/builderConfig.test.ts
index 32a1bec58131..3f8c8a20b2ab 100644
--- a/packages/api/test/unit/keymanager/builderConfig.test.ts
+++ b/packages/api/test/unit/keymanager/builderConfig.test.ts
@@ -19,4 +19,10 @@ describe("builderConfigDataFromJson", () => {
       );
     }
   });
+
+  it("suggests the ASCII form of a builder url with an internationalized hostname", () => {
+    expect(() => builderConfigDataFromJson({builders: [{url: "https://platåberget.dev"}]})).toThrow(
+      "use https://xn--platberget-45a.dev/ instead"
+    );
+  });
 });
diff --git a/packages/cli/src/util/proposerConfig.ts b/packages/cli/src/util/proposerConfig.ts
index c38d9f8de1dd..27e348b9d110 100644
--- a/packages/cli/src/util/proposerConfig.ts
+++ b/packages/cli/src/util/proposerConfig.ts
@@ -3,7 +3,7 @@ import path from "node:path";
 import {routes} from "@lodestar/api";
 import {BuilderEntryConfig, builderConfigDataFromJson} from "@lodestar/api/keymanager";
 import {MAX_BUILDER_AUTH_DATA_SIZE, MAX_BUILDER_ENTRIES, MAX_BUILDER_URL_SIZE} from "@lodestar/params";
-import {fromHex, isValidAsciiHttpUrl, toHex} from "@lodestar/utils";
+import {fromHex, isValidAsciiHttpUrl, toAsciiHttpUrl, toHex} from "@lodestar/utils";
 import {ValidatorProposerConfig} from "@lodestar/validator";
 import {parseFeeRecipient} from "./feeRecipient.js";
 import {readFile} from "./file.js";
@@ -252,7 +252,8 @@ export function parseBuilderUrls(urls?: string[]): BuilderEntryConfig[] | undefi
     const url = fragmentIndex === -1 ? value : value.slice(0, fragmentIndex);
     const authData = fragmentIndex === -1 ? undefined : value.slice(fragmentIndex + 1);
     if (!isValidAsciiHttpUrl(url)) {
-      throw Error(`Invalid builder url: ${url}`);
+      const asciiUrl = toAsciiHttpUrl(url);
+      throw Error(`Invalid builder url: ${url}` + (asciiUrl !== null ? `, use ${asciiUrl} instead` : ""));
     }
     if (Buffer.byteLength(url, "utf8") > MAX_BUILDER_URL_SIZE) {
       throw Error(`Invalid builder url, must not exceed ${MAX_BUILDER_URL_SIZE} bytes: ${url}`);
diff --git a/packages/cli/test/unit/validator/parseBuilderUrls.test.ts b/packages/cli/test/unit/validator/parseBuilderUrls.test.ts
index ac7305160f52..c77d33f8cdcf 100644
--- a/packages/cli/test/unit/validator/parseBuilderUrls.test.ts
+++ b/packages/cli/test/unit/validator/parseBuilderUrls.test.ts
@@ -25,6 +25,7 @@ describe("validator / parseBuilderUrls", () => {
   it("rejects invalid urls and auth data", () => {
     expect(() => parseBuilderUrls(["builder.example.com"])).toThrow(/Invalid builder url/);
     expect(() => parseBuilderUrls(["https://builder.example.com/é"])).toThrow(/Invalid builder url/);
+    expect(() => parseBuilderUrls(["https://platåberget.dev"])).toThrow("use https://xn--platberget-45a.dev/ instead");
     expect(() => parseBuilderUrls(["https://builder.example.com#"])).toThrow(/auth data/);
     expect(() => parseBuilderUrls(["https://builder.example.com#0x"])).toThrow(/auth data/);
     expect(() => parseBuilderUrls(["https://builder.example.com#secret"])).toThrow(/auth data/);
diff --git a/packages/utils/src/url.ts b/packages/utils/src/url.ts
index e53f44019d83..6acc898abe35 100644
--- a/packages/utils/src/url.ts
+++ b/packages/utils/src/url.ts
@@ -31,6 +31,16 @@ export function isValidAsciiHttpUrl(urlStr: string): boolean {
   return isValidHttpUrl(urlStr);
 }
 
+/** Return the ASCII form of an HTTP(S) URL with its hostname in punycode, or null if it has none. */
+export function toAsciiHttpUrl(urlStr: string): string | null {
+  if (!isValidHttpUrl(urlStr)) {
+    return null;
+  }
+
+  const asciiUrl = new URL(urlStr).href;
+  return isValidAsciiHttpUrl(asciiUrl) ? asciiUrl : null;
+}
+
 /**
  * Sanitize URL to prevent leaking user credentials in logs or metrics
  *
diff --git a/packages/utils/test/unit/url.test.ts b/packages/utils/test/unit/url.test.ts
new file mode 100644
index 000000000000..eacb0653dbf3
--- /dev/null
+++ b/packages/utils/test/unit/url.test.ts
@@ -0,0 +1,18 @@
+import {describe, expect, it} from "vitest";
+import {toAsciiHttpUrl} from "../../src/url.js";
+
+describe("toAsciiHttpUrl", () => {
+  const testCases: {url: string; expected: string | null}[] = [
+    {url: "https://platåberget.dev", expected: "https://xn--platberget-45a.dev/"},
+    {url: "https://builder.example.com/é?x=1", expected: "https://builder.example.com/%C3%A9?x=1"},
+    {url: "https://builder.example.com:8080/path", expected: "https://builder.example.com:8080/path"},
+    {url: "ftp://builder.example.com", expected: null},
+    {url: "builder.example.com", expected: null},
+  ];
+
+  for (const {url, expected} of testCases) {
+    it(url, () => {
+      expect(toAsciiHttpUrl(url)).toBe(expected);
+    });
+  }
+});