diff --git a/src/get-app-authentication.ts b/src/get-app-authentication.ts
index bb585ce18..460fc8060 100644
--- a/src/get-app-authentication.ts
+++ b/src/get-app-authentication.ts
@@ -1,6 +1,7 @@
 import { githubAppJwt } from "universal-github-app-jwt";
 
 import type { AppAuthentication, State } from "./types";
+import { validatePrivatekeyContent } from "./validate-pk-content";
 
 export async function getAppAuthentication({
   appId,
@@ -21,9 +22,9 @@ export async function getAppAuthentication({
       expiresAt: new Date(appAuthentication.expiration * 1000).toISOString(),
     };
   } catch (error) {
-    if (privateKey === "-----BEGIN RSA PRIVATE KEY-----") {
+    if (!validatePrivatekeyContent(privateKey)) {
       throw new Error(
-        "The 'privateKey` option contains only the first line '-----BEGIN RSA PRIVATE KEY-----'. If you are setting it using a `.env` file, make sure it is set on a single line with newlines replaced by '\n'"
+        "[@octokit/auth-app] privateKey only contains the first line. Try replacing line breaks with \n if you are setting it as multiline string (e.g. environment variable)"
       );
     } else {
       throw error;
diff --git a/src/index.ts b/src/index.ts
index 9d9c72a9a..6cc2bc0f3 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -7,6 +7,7 @@ import { hook } from "./hook";
 import { getCache } from "./cache";
 import type { AuthInterface, State, StrategyOptions } from "./types";
 import { VERSION } from "./version";
+import { validatePrivatekeyContent } from "./validate-pk-content";
 
 export { createOAuthUserAuth } from "@octokit/auth-oauth-user";
 export type {
@@ -45,6 +46,14 @@ export function createAppAuth(options: StrategyOptions): AuthInterface {
     );
   }
 
+  // This check ensures that private key contains the actual content
+  // specifically when set using environment variables as multiline string.
+  if (!validatePrivatekeyContent(options.privateKey)) {
+    throw new Error(
+      "[@octokit/auth-app] privateKey only contains the first line. Try replacing line breaks with \n if you are setting it as multiline string (e.g. environment variable)"
+    );
+  }
+
   const log = Object.assign(
     {
       warn: console.warn.bind(console),
diff --git a/src/validate-pk-content.ts b/src/validate-pk-content.ts
new file mode 100644
index 000000000..eab66b342
--- /dev/null
+++ b/src/validate-pk-content.ts
@@ -0,0 +1,23 @@
+export function validatePrivatekeyContent(privateKey: string) {
+  // first check
+  if (!privateKey.startsWith("----") && !privateKey.endsWith("----")) {
+    return false;
+  }
+
+  // second check
+  const pk: string[] = privateKey.trim().split(" ");
+  if (pk.length > 1) {
+    const protocol = pk[1];
+    const begin = `-----BEGIN ${protocol} PRIVATE KEY-----`;
+    const end = `-----END ${protocol} PRIVATE KEY----`;
+
+    if (!(privateKey.includes(begin) && privateKey.includes(end))) {
+      return false;
+    }
+  } else {
+    // there is no whitespace inside private key content and still invalid
+    return false;
+  }
+
+  return true;
+}
diff --git a/test/index.test.ts b/test/index.test.ts
index b31a3bb29..b37646822 100644
--- a/test/index.test.ts
+++ b/test/index.test.ts
@@ -110,7 +110,7 @@ test("throws if incomplete Private Key is provided", async () => {
 
   await expect(auth({ type: "app" })).rejects.toEqual(
     new Error(
-      "The 'privateKey` option contains only the first line '-----BEGIN RSA PRIVATE KEY-----'. If you are setting it using a `.env` file, make sure it is set on a single line with newlines replaced by '\n'"
+      "[@octokit/auth-app] privateKey only contains the first line. Try replacing line breaks with \n if you are setting it as multiline string (e.g. environment variable)"
     )
   );
 });
diff --git a/test/validate-pk-content.test.ts b/test/validate-pk-content.test.ts
new file mode 100644
index 000000000..cc9d1acb4
--- /dev/null
+++ b/test/validate-pk-content.test.ts
@@ -0,0 +1,52 @@
+import { validatePrivatekeyContent } from "../src/validate-pk-content";
+
+const VALID_MULTI_LINE_PRIVATE_KEY = `-----BEGIN RSA PRIVATE KEY-----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+-----END RSA PRIVATE KEY-----`;
+
+const VALID_SINGLE_LINE_PRIVATE_KEY =
+  "-----BEGIN RSA PRIVATE KEY-----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\nx//0u+zd/R/QRUzLOw4N72/Hu+UG6MNt5iDZFCtapRaKt6OvSBwy8w==\n-----END RSA PRIVATE KEY----";
+
+test("valid multi-line private key", async () => {
+  const isValid = validatePrivatekeyContent(VALID_MULTI_LINE_PRIVATE_KEY);
+  await expect(isValid).toStrictEqual(true);
+});
+
+test("valid single-line private key", async () => {
+  const isValid = validatePrivatekeyContent(VALID_SINGLE_LINE_PRIVATE_KEY);
+  await expect(isValid).toStrictEqual(true);
+});
+
+test("invalid private key INVALID_PRIVATE_KEY", async () => {
+  const isValid = validatePrivatekeyContent("INVALID_PRIVATE_KEY");
+  await expect(isValid).toStrictEqual(false);
+});
+
+test("invalid private key -----BEGIN RSA PRIVATE KEY-----", async () => {
+  const isValid = validatePrivatekeyContent("-----BEGIN RSA PRIVATE KEY-----");
+  await expect(isValid).toStrictEqual(false);
+});