diff --git a/lib/build/bundle.js b/lib/build/bundle.js
index b8ef64c5..a08086a3 100644
--- a/lib/build/bundle.js
+++ b/lib/build/bundle.js
@@ -364,10 +364,7 @@ exports.Bundle = class {
       }
 
       let mapFileName = bundleFileName + '.map';
-      let mapSourceRoot = path.posix.relative(
-        path.posix.join(process.cwd(), platform.output),
-        process.cwd()
-      );
+      let mapSourceRoot = calculateRelativeSourceMapsRoot(process.cwd(), platform.output);
 
       logger.info(`Writing ${bundleFileName}...`);
 
@@ -531,3 +528,20 @@ function uniqueBy(collection, key) {
     return seen.hasOwnProperty(k) ? false : (seen[k] = true);
   });
 }
+
+/**
+ * Returns a POSIX-style relative path from `outputDir` back to `projectRoot`.
+ * Works on Windows, macOS and Linux.
+ *
+ * @param {string} projectRootDir - The root directory of the project.
+ * @param {string} outputDir - The output directory where the files are generated.
+ * @returns {string} A POSIX-style relative path from `outputDir` to `projectRoot`.
+ */
+function calculateRelativeSourceMapsRoot(projectRootDir, outputDir){
+  const absoluteProjectRootDir = path.resolve(projectRootDir.split('\\').join('/'));
+  const absoluteOutputDir = path.resolve(absoluteProjectRootDir, outputDir.split('\\').join('/'));
+
+  return path.relative(absoluteOutputDir, absoluteProjectRootDir).split('\\').join('/');
+}
+
+exports._calculateRelativeSourceMapsRoot = calculateRelativeSourceMapsRoot;
diff --git a/spec/lib/build/bundle.spec.js b/spec/lib/build/bundle.spec.js
index df972f42..9ea4288a 100644
--- a/spec/lib/build/bundle.spec.js
+++ b/spec/lib/build/bundle.spec.js
@@ -1,5 +1,6 @@
 const BundlerMock = require('../../mocks/bundler');
 const Bundle = require('../../../lib/build/bundle').Bundle;
+const _calculateRelativeSourceMapsRoot = require('../../../lib/build/bundle')._calculateRelativeSourceMapsRoot;
 const CLIOptionsMock = require('../../mocks/cli-options');
 const DependencyDescription = require('../../../lib/build/dependency-description').DependencyDescription;
 const SourceInclusion = require('../../../lib/build/source-inclusion').SourceInclusion;
@@ -459,3 +460,45 @@ describe('the Bundle module', () => {
     });
   });
 });
+
+describe('function _calculateRelativeSourceMapsRoot', () => {
+  const testCases = [
+    // Basic UNIX cases
+    { projectDir: '/usr/home/my-app', outputDir: './dist', expected: '..' },
+    { projectDir: '/usr/home/my-app/', outputDir: './dist/', expected: '..' },
+    { projectDir: '/usr/home/my-app', outputDir: 'dist', expected: '..' },
+    { projectDir: '/usr/home/my-app/', outputDir: 'dist/', expected: '..' },
+    // Basic Windows cases
+    { projectDir: 'C:/My Documents/MyApp', outputDir: './dist', expected: '..' },
+    { projectDir: 'C:/My Documents/MyApp/', outputDir: './dist/', expected: '..' },
+    { projectDir: 'C:/My Documents/MyApp', outputDir: 'dist', expected: '..' },
+    { projectDir: 'C:/My Documents/MyApp/', outputDir: 'dist/', expected: '..' },
+    // Basic Windows cases with backslashes
+    { projectDir: 'C:\\My Documents\\MyApp', outputDir: '.\\dist', expected: '..' },
+    { projectDir: 'C:\\My Documents\\MyApp\\', outputDir: '.\\dist\\', expected: '..' },
+    { projectDir: 'C:\\My Documents\\MyApp', outputDir: 'dist', expected: '..' },
+    { projectDir: 'C:\\My Documents\\MyApp\\', outputDir: 'dist\\', expected: '..' },
+    // Windows mixed slashes
+    { projectDir: 'C:\\My Documents\\MyApp', outputDir: './dist', expected: '..' },
+    { projectDir: 'C:\\My Documents\\MyApp\\', outputDir: './dist/', expected: '..' },
+    { projectDir: 'C:/My Documents/MyApp/', outputDir: 'dist\\', expected: '..' },
+    // Output directory outside of project root
+    { projectDir: '/usr/home/my-app', outputDir: '../wwwroot/scripts', expected: '../../my-app' },
+    { projectDir: 'C:\\My Documents\\MyApp', outputDir: '../wwwroot/scripts', expected: '../../MyApp' },
+    // Relative project root paths, basic cases
+    { projectDir: './my-app', outputDir: './dist', expected: '..' },
+    { projectDir: 'my-app', outputDir: 'dist', expected: '..' },
+    { projectDir: '.\\MyApp', outputDir: '.\\dist', expected: '..' },
+    { projectDir: 'MyApp\\', outputDir: 'dist\\', expected: '..' },
+    // Relative project root paths, output directory outside of project root
+    { projectDir: './my-app', outputDir: '../wwwroot/scripts', expected: '../../my-app' },
+    { projectDir: '.\\MyApp\\', outputDir: '..\\wwwroot\\scripts\\', expected: '../../MyApp' }
+  ];
+
+  testCases.forEach(({ projectDir, outputDir, expected }) => {
+    it(`returns "${expected}" for projectDir "${projectDir}" and outputDir "${outputDir}"`, () => {
+      const result = _calculateRelativeSourceMapsRoot(projectDir, outputDir);
+      expect(result).toBe(expected);
+    });
+  });
+});