diff --git a/.changeset/dirty-trains-yawn.md b/.changeset/dirty-trains-yawn.md
new file mode 100644
index 000000000000..aaf62059f19b
--- /dev/null
+++ b/.changeset/dirty-trains-yawn.md
@@ -0,0 +1,5 @@
+---
+'astro': patch
+---
+
+Fix relative config handling with the `--config` flag
diff --git a/packages/astro/src/core/config.ts b/packages/astro/src/core/config.ts
index c6b09b123a07..5231f5ffa45a 100644
--- a/packages/astro/src/core/config.ts
+++ b/packages/astro/src/core/config.ts
@@ -7,7 +7,7 @@ import path from 'path';
 import { pathToFileURL, fileURLToPath } from 'url';
 import { mergeConfig as mergeViteConfig } from 'vite';
 import { z } from 'zod';
-import load from '@proload/core';
+import load, { ProloadError } from '@proload/core';
 import loadTypeScript from '@proload/plugin-tsm';
 import postcssrc from 'postcss-load-config';
 import { arraify, isObject } from './util.js';
@@ -379,11 +379,20 @@ export async function loadConfig(configOptions: LoadConfigOptions): Promise<Astr
 
 	if (flags?.config) {
 		userConfigPath = /^\.*\//.test(flags.config) ? flags.config : `./${flags.config}`;
-		userConfigPath = fileURLToPath(new URL(userConfigPath, pathToFileURL(root)));
+		userConfigPath = fileURLToPath(new URL(userConfigPath, appendForwardSlash(pathToFileURL(root).toString())));
 	}
+
 	// Automatically load config file using Proload
 	// If `userConfigPath` is `undefined`, Proload will search for `astro.config.[cm]?[jt]s`
-	const config = await load('astro', { mustExist: false, cwd: root, filePath: userConfigPath });
+	let config;
+	try {
+		config = await load('astro', { mustExist: !!userConfigPath, cwd: root, filePath: userConfigPath });
+	} catch (err) {
+		if (err instanceof ProloadError && flags.config) {
+			throw new Error(`Unable to resolve --config "${flags.config}"! Does the file exist?`);
+		}
+		throw err;
+	}
 	if (config) {
 		userConfig = config.value;
 	}
diff --git a/packages/astro/test/config.test.js b/packages/astro/test/config.test.js
index 1f255097c142..fe25e2f9eff5 100644
--- a/packages/astro/test/config.test.js
+++ b/packages/astro/test/config.test.js
@@ -64,6 +64,60 @@ describe('config', () => {
 		});
 	});
 
+	describe('relative path', () => {
+		it('can be passed via relative --config', async () => {
+			const projectRootURL = new URL('./fixtures/astro-basic/', import.meta.url);
+			const configFileURL = 'my-config.mjs';
+			const { local } = await cliServerLogSetup([
+				'--root',
+				fileURLToPath(projectRootURL),
+				'--config',
+				configFileURL,
+			]);
+
+			const localURL = new URL(local);
+			expect(localURL.port).to.equal('8080');
+		});
+	})
+
+	describe('relative path with leading ./', () => {
+		it('can be passed via relative --config', async () => {
+			const projectRootURL = new URL('./fixtures/astro-basic/', import.meta.url);
+			const configFileURL = './my-config.mjs';
+			const { local } = await cliServerLogSetup([
+				'--root',
+				fileURLToPath(projectRootURL),
+				'--config',
+				configFileURL,
+			]);
+
+			const localURL = new URL(local);
+			expect(localURL.port).to.equal('8080');
+		});
+	})
+
+	describe('incorrect path', () => {
+		it('fails and exits when config does not exist', async () => {
+			const projectRootURL = new URL('./fixtures/astro-basic/', import.meta.url);
+			const configFileURL = './does-not-exist.mjs';
+			let exit = 0;
+			try {
+				await cliServerLogSetup([
+					'--root',
+					fileURLToPath(projectRootURL),
+					'--config',
+					configFileURL,
+				]);
+			} catch (e) {
+				if (e.message.includes('Unable to resolve --config')) {
+					exit = 1;
+				}
+			}
+			
+			expect(exit).to.equal(1, "Throws helpful error message when --config does not exist");
+		});
+	})
+
 	describe('port', () => {
 		it('can be specified in astro.config.mjs', async () => {
 			expect(portFixture.config.server.port).to.deep.equal(5006);
diff --git a/packages/astro/test/fixtures/astro-basic/my-config.mjs b/packages/astro/test/fixtures/astro-basic/my-config.mjs
new file mode 100644
index 000000000000..7ed24f69f250
--- /dev/null
+++ b/packages/astro/test/fixtures/astro-basic/my-config.mjs
@@ -0,0 +1,5 @@
+export default {
+  server: {
+    port: 8080,
+  },
+}
diff --git a/packages/astro/test/test-utils.js b/packages/astro/test/test-utils.js
index ab04f96046b6..b850486c3ee2 100644
--- a/packages/astro/test/test-utils.js
+++ b/packages/astro/test/test-utils.js
@@ -151,19 +151,32 @@ export function cli(/** @type {string[]} */ ...args) {
 
 export async function parseCliDevStart(proc) {
 	let stdout = '';
+	let stderr = '';
 
 	for await (const chunk of proc.stdout) {
 		stdout += chunk;
-
 		if (chunk.includes('Local')) break;
 	}
+	if (!stdout) {
+		for await (const chunk of proc.stderr) {
+			stderr += chunk;
+			break;
+		}
+	}
 
 	proc.kill();
 	stdout = stripAnsi(stdout);
+	stderr = stripAnsi(stderr);
+
+	if (stderr) {
+		throw new Error(stderr);
+	}
+	
 	const messages = stdout
 		.split('\n')
 		.filter((ln) => !!ln.trim())
 		.map((ln) => ln.replace(/[🚀┃]/g, '').replace(/\s+/g, ' ').trim());
+
 	return { messages };
 }
 
@@ -172,11 +185,8 @@ export async function cliServerLogSetup(flags = [], cmd = 'dev') {
 
 	const { messages } = await parseCliDevStart(proc);
 
-	const localRaw = (messages[1] ?? '').includes('Local') ? messages[1] : undefined;
-	const networkRaw = (messages[2] ?? '').includes('Network') ? messages[2] : undefined;
-
-	const local = localRaw?.replace(/Local\s*/g, '');
-	const network = networkRaw?.replace(/Network\s*/g, '');
+	const local = messages.find(msg => msg.includes('Local'))?.replace(/Local\s*/g, '');
+	const network = messages.find(msg => msg.includes('Network'))?.replace(/Network\s*/g, '');
 
 	return { local, network };
 }