diff --git a/lib/entry-points.js b/lib/entry-points.js
index c91850e7a..ab20d259a 100644
--- a/lib/entry-points.js
+++ b/lib/entry-points.js
@@ -151053,7 +151053,7 @@ function getInvalidConfigFileMessage(configFile, messages) {
 }
 function getConfigFileRepoFormatInvalidMessage(configFile) {
   let error3 = `The configuration file "${configFile}" is not a supported remote file reference.`;
-  error3 += " Expected format [<owner>/]<repository>[/<file-path>][@<ref>]";
+  error3 += " Expected format [<owner>/]<repository>[@<ref>][:<file-path>]";
   return error3;
 }
 function getConfigFileFormatInvalidMessage(configFile) {
@@ -151447,9 +151447,28 @@ function getDefaultOwner(env) {
   }
   return nwoParts[0].trim();
 }
+var OLD_REMOTE_ADDRESS_FORMAT = new RegExp(
+  "(?<owner>[^/]+)/(?<repo>[^/]+)/(?<path>[^@]+)@(?<ref>.*)"
+);
+function parseOldRemoteFileAddress(input) {
+  const pieces = OLD_REMOTE_ADDRESS_FORMAT.exec(input);
+  if (pieces?.groups === void 0 || pieces.length < 5) {
+    return new Failure(void 0);
+  }
+  return new Success({
+    owner: pieces.groups.owner.trim(),
+    repo: pieces.groups.repo.trim(),
+    path: pieces.groups.path.trim(),
+    ref: pieces.groups.ref.trim()
+  });
+}
 function parseRemoteFileAddress(env, configFile) {
+  const oldFormatAddressResult = parseOldRemoteFileAddress(configFile);
+  if (oldFormatAddressResult.isSuccess()) {
+    return oldFormatAddressResult.value;
+  }
   const format = new RegExp(
-    "^((?<owner>[^/]+)/)?(?<repo>[^/@]+)(/(?<path>[^@]+))?(@(?<ref>.*))?$"
+    "^((?<owner>[^:@/]+)/)?(?<repo>[^:@/]+)(@(?<ref>[^:]+))?(:(?<path>.+))?$"
   );
   const pieces = format.exec(configFile.trim());
   const repo = pieces?.groups?.repo?.trim();
diff --git a/src/config/remote-file.test.ts b/src/config/remote-file.test.ts
index 36b68fd55..f0ef1c13c 100644
--- a/src/config/remote-file.test.ts
+++ b/src/config/remote-file.test.ts
@@ -15,6 +15,7 @@ import {
 test("parseRemoteFileAddress accepts full remote addresses", async (t) => {
   const env = getTestEnv();
 
+  // Old format.
   t.deepEqual(parseRemoteFileAddress(env, "owner/repo/path@ref"), {
     owner: "owner",
     repo: "repo",
@@ -86,6 +87,79 @@ test("parseRemoteFileAddress accepts full remote addresses", async (t) => {
       ref: "ref/feature",
     } satisfies RemoteFileAddress,
   );
+
+  // New format.
+  t.deepEqual(parseRemoteFileAddress(env, "owner/repo@ref:path"), {
+    owner: "owner",
+    repo: "repo",
+    path: "path",
+    ref: "ref",
+  } satisfies RemoteFileAddress);
+
+  t.deepEqual(parseRemoteFileAddress(env, "owner  /repo@ref:path"), {
+    owner: "owner",
+    repo: "repo",
+    path: "path",
+    ref: "ref",
+  } satisfies RemoteFileAddress);
+
+  t.deepEqual(parseRemoteFileAddress(env, "owner/   repo@ref:path"), {
+    owner: "owner",
+    repo: "repo",
+    path: "path",
+    ref: "ref",
+  } satisfies RemoteFileAddress);
+
+  t.deepEqual(parseRemoteFileAddress(env, "owner/repo   @ref:path"), {
+    owner: "owner",
+    repo: "repo",
+    path: "path",
+    ref: "ref",
+  } satisfies RemoteFileAddress);
+
+  t.deepEqual(parseRemoteFileAddress(env, "owner/repo@   ref:path"), {
+    owner: "owner",
+    repo: "repo",
+    path: "path",
+    ref: "ref",
+  } satisfies RemoteFileAddress);
+
+  t.deepEqual(parseRemoteFileAddress(env, "owner/repo@ref   :path"), {
+    owner: "owner",
+    repo: "repo",
+    path: "path",
+    ref: "ref",
+  } satisfies RemoteFileAddress);
+
+  t.deepEqual(parseRemoteFileAddress(env, "owner/repo@ref:   path"), {
+    owner: "owner",
+    repo: "repo",
+    path: "path",
+    ref: "ref",
+  } satisfies RemoteFileAddress);
+
+  t.deepEqual(
+    parseRemoteFileAddress(env, "owner/repo@ref/feature:path/to/codeql.yml"),
+    {
+      owner: "owner",
+      repo: "repo",
+      path: "path/to/codeql.yml",
+      ref: "ref/feature",
+    } satisfies RemoteFileAddress,
+  );
+
+  t.deepEqual(
+    parseRemoteFileAddress(
+      env,
+      "  owner/repo@ref/feature:path/to/codeql.yml  ",
+    ),
+    {
+      owner: "owner",
+      repo: "repo",
+      path: "path/to/codeql.yml",
+      ref: "ref/feature",
+    } satisfies RemoteFileAddress,
+  );
 });
 
 test("parseRemoteFileAddress accepts remote address without an owner", async (t) => {
@@ -96,6 +170,13 @@ test("parseRemoteFileAddress accepts remote address without an owner", async (t)
     .withArgs(ActionsEnvVars.GITHUB_REPOSITORY)
     .returns(`${owner}/current-repo`);
 
+  t.deepEqual(parseRemoteFileAddress(env, "repo@ref:path.yml"), {
+    owner,
+    repo: "repo",
+    path: "path.yml",
+    ref: "ref",
+  } satisfies RemoteFileAddress);
+
   t.deepEqual(parseRemoteFileAddress(env, "repo@ref"), {
     owner,
     repo: "repo",
@@ -103,6 +184,13 @@ test("parseRemoteFileAddress accepts remote address without an owner", async (t)
     ref: "ref",
   } satisfies RemoteFileAddress);
 
+  t.deepEqual(parseRemoteFileAddress(env, "repo:path.yml"), {
+    owner,
+    repo: "repo",
+    path: "path.yml",
+    ref: DEFAULT_CONFIG_FILE_REF,
+  } satisfies RemoteFileAddress);
+
   t.deepEqual(parseRemoteFileAddress(env, "repo"), {
     owner,
     repo: "repo",
@@ -142,14 +230,7 @@ test("parseRemoteFileAddress accepts remote address without a path", async (t) =
 test("parseRemoteFileAddress accepts remote address without a ref", async (t) => {
   const env = getTestEnv();
 
-  t.deepEqual(parseRemoteFileAddress(env, "owner/repo/path"), {
-    owner: "owner",
-    repo: "repo",
-    path: "path",
-    ref: DEFAULT_CONFIG_FILE_REF,
-  } satisfies RemoteFileAddress);
-
-  t.deepEqual(parseRemoteFileAddress(env, "owner/repo/path@"), {
+  t.deepEqual(parseRemoteFileAddress(env, "owner/repo:path"), {
     owner: "owner",
     repo: "repo",
     path: "path",
@@ -171,10 +252,43 @@ test("parseRemoteFileAddress rejects invalid values", async (t) => {
   t.throws(() => parseRemoteFileAddress(env, "repo//absolute"), {
     instanceOf: ConfigurationError,
   });
+  t.throws(() => parseRemoteFileAddress(env, "repo:/absolute"), {
+    instanceOf: ConfigurationError,
+  });
   t.throws(() => parseRemoteFileAddress(env, "/repo@ref"), {
     instanceOf: ConfigurationError,
   });
   t.throws(() => parseRemoteFileAddress(env, "   /repo@ref"), {
     instanceOf: ConfigurationError,
   });
+  t.throws(() => parseRemoteFileAddress(env, "repo@"), {
+    instanceOf: ConfigurationError,
+  });
+  t.throws(() => parseRemoteFileAddress(env, "repo:"), {
+    instanceOf: ConfigurationError,
+  });
+  t.throws(() => parseRemoteFileAddress(env, "repo/"), {
+    instanceOf: ConfigurationError,
+  });
+  t.throws(() => parseRemoteFileAddress(env, "/repo"), {
+    instanceOf: ConfigurationError,
+  });
+  t.throws(() => parseRemoteFileAddress(env, ":path"), {
+    instanceOf: ConfigurationError,
+  });
+  t.throws(() => parseRemoteFileAddress(env, "@ref"), {
+    instanceOf: ConfigurationError,
+  });
+  t.throws(() => parseRemoteFileAddress(env, "@ref:path"), {
+    instanceOf: ConfigurationError,
+  });
+  t.throws(() => parseRemoteFileAddress(env, "owner/@ref:path"), {
+    instanceOf: ConfigurationError,
+  });
+  t.throws(() => parseRemoteFileAddress(env, "owner/@ref"), {
+    instanceOf: ConfigurationError,
+  });
+  t.throws(() => parseRemoteFileAddress(env, "owner/:path"), {
+    instanceOf: ConfigurationError,
+  });
 });
diff --git a/src/config/remote-file.ts b/src/config/remote-file.ts
index a4f8ea890..af68380e1 100644
--- a/src/config/remote-file.ts
+++ b/src/config/remote-file.ts
@@ -1,7 +1,7 @@
 import { ActionsEnvVars } from "../actions-util";
 import { Env } from "../environment";
 import * as errorMessages from "../error-messages";
-import { ConfigurationError } from "../util";
+import { ConfigurationError, Failure, Result, Success } from "../util";
 
 /** Represents remote file addresses. */
 export interface RemoteFileAddress {
@@ -37,6 +37,38 @@ function getDefaultOwner(env: Env): string {
   return nwoParts[0].trim();
 }
 
+/**
+ * The old remote address format that's always been supported for the `config-file` input.
+ * All the components are required. Unchanged from the previous implementation.
+ */
+const OLD_REMOTE_ADDRESS_FORMAT = new RegExp(
+  "(?<owner>[^/]+)/(?<repo>[^/]+)/(?<path>[^@]+)@(?<ref>.*)",
+);
+
+/**
+ * Attempts to parse `input` as a `RemoteFileAddress` using the old format.
+ *
+ * @param input The input to try and parse.
+ * @returns A `RemoteFileAddress` value if successful or `undefined` otherwise.
+ */
+function parseOldRemoteFileAddress(
+  input: string,
+): Result<RemoteFileAddress, undefined> {
+  const pieces = OLD_REMOTE_ADDRESS_FORMAT.exec(input);
+
+  // 5 = 4 groups + the whole expression
+  if (pieces?.groups === undefined || pieces.length < 5) {
+    return new Failure(undefined);
+  }
+
+  return new Success({
+    owner: pieces.groups.owner.trim(),
+    repo: pieces.groups.repo.trim(),
+    path: pieces.groups.path.trim(),
+    ref: pieces.groups.ref.trim(),
+  });
+}
+
 /**
  * Attempts to parse `configFile` into an array of `RemoteFileAddress` components.
  *
@@ -49,9 +81,17 @@ export function parseRemoteFileAddress(
   env: Env,
   configFile: string,
 ): RemoteFileAddress {
+  // Try to parse the input using the old format. If successful, return the
+  // resulting `RemoteFileAddress`. Otherwise, continue using the new format.
+  const oldFormatAddressResult = parseOldRemoteFileAddress(configFile);
+
+  if (oldFormatAddressResult.isSuccess()) {
+    return oldFormatAddressResult.value;
+  }
+
   // retrieve the various parts of the config location, and ensure they're present
   const format = new RegExp(
-    "^((?<owner>[^/]+)/)?(?<repo>[^/@]+)(/(?<path>[^@]+))?(@(?<ref>.*))?$",
+    "^((?<owner>[^:@/]+)/)?(?<repo>[^:@/]+)(@(?<ref>[^:]+))?(:(?<path>.+))?$",
   );
   const pieces = format.exec(configFile.trim());
 
@@ -59,6 +99,9 @@ export function parseRemoteFileAddress(
 
   // Check that the regular expression matched and that we have at least the repo name.
   if (!pieces?.groups || !repo || repo.length === 0) {
+    // Neither the old format nor the new format worked. Throw an error that
+    // explains the format we accept. We only mention the new format, since that's
+    // what we want to be used going forward.
     throw new ConfigurationError(
       errorMessages.getConfigFileRepoFormatInvalidMessage(configFile),
     );
diff --git a/src/error-messages.ts b/src/error-messages.ts
index b45d9562b..377310933 100644
--- a/src/error-messages.ts
+++ b/src/error-messages.ts
@@ -34,7 +34,7 @@ export function getConfigFileRepoFormatInvalidMessage(
   configFile: string,
 ): string {
   let error = `The configuration file "${configFile}" is not a supported remote file reference.`;
-  error += " Expected format [<owner>/]<repository>[/<file-path>][@<ref>]";
+  error += " Expected format [<owner>/]<repository>[@<ref>][:<file-path>]";
 
   return error;
 }