diff --git a/src/compiler/preprocess/index.ts b/src/compiler/preprocess/index.ts
index 1de41cf9b..b51b67bb2 100644
--- a/src/compiler/preprocess/index.ts
+++ b/src/compiler/preprocess/index.ts
@@ -83,8 +83,81 @@ async function replace_async(
 	return out.concat(final_content);
 }
 
-/** 
- * Convert a preprocessor output and its leading prefix and trailing suffix into StringWithSourceMap  
+/**
+ * Import decoded sourcemap from mozilla/source-map/SourceMapGenerator
+ * Forked from source-map/lib/source-map-generator.js
+ * from methods _serializeMappings and toJSON.
+ * We cannot use source-map.d.ts types, because we access hidden properties.
+ */
+function decoded_sourcemap_from_generator(generator: any) {
+	let previous_generated_line = 1;
+	const converted_mappings = [[]];
+	let result_line;
+	let result_segment;
+	let mapping;
+
+	const source_idx = generator._sources.toArray()
+		.reduce((acc, val, idx) => (acc[val] = idx, acc), {});
+
+	const name_idx = generator._names.toArray()
+		.reduce((acc, val, idx) => (acc[val] = idx, acc), {});
+
+	const mappings = generator._mappings.toArray();
+	result_line = converted_mappings[0];
+
+	for (let i = 0, len = mappings.length; i < len; i++) {
+		mapping = mappings[i];
+
+		if (mapping.generatedLine > previous_generated_line) {
+			while (mapping.generatedLine > previous_generated_line) {
+				converted_mappings.push([]);
+				previous_generated_line++;
+			}
+			result_line = converted_mappings[mapping.generatedLine - 1]; // line is one-based
+		} else if (i > 0) {
+			const previous_mapping = mappings[i - 1];
+			if (
+				// sorted by selectivity
+				mapping.generatedColumn === previous_mapping.generatedColumn &&
+				mapping.originalColumn === previous_mapping.originalColumn &&
+				mapping.name === previous_mapping.name &&
+				mapping.generatedLine === previous_mapping.generatedLine &&
+				mapping.originalLine === previous_mapping.originalLine &&
+				mapping.source === previous_mapping.source
+		) {
+				continue;
+			}
+		}
+		result_line.push([mapping.generatedColumn]);
+		result_segment = result_line[result_line.length - 1];
+
+		if (mapping.source != null) {
+			result_segment.push(...[
+				source_idx[mapping.source],
+				mapping.originalLine - 1, // line is one-based
+				mapping.originalColumn
+			]);
+			if (mapping.name != null) {
+				result_segment.push(name_idx[mapping.name]);
+			}
+		}
+	}
+
+	const map = {
+		version: generator._version,
+		sources: generator._sources.toArray(),
+		names: generator._names.toArray(),
+		mappings: converted_mappings
+	};
+	if (generator._file != null) {
+		(map as any).file = generator._file;
+	}
+	// not needed: map.sourcesContent and map.sourceRoot
+	return map;
+}
+
+/**
+ * Convert a preprocessor output and its leading prefix and trailing suffix into StringWithSourceMap
  */
 function get_replacement(
 	filename: string,
@@ -109,6 +182,10 @@ function get_replacement(
 		if (typeof(decoded_map.mappings) === 'string') {
 			decoded_map.mappings = decode_mappings(decoded_map.mappings);
 		}
+		if ((decoded_map as any)._mappings && decoded_map.constructor.name === 'SourceMapGenerator') {
+			// import decoded sourcemap from mozilla/source-map/SourceMapGenerator
+			decoded_map = decoded_sourcemap_from_generator(decoded_map);
+		}
 		sourcemap_add_offset(decoded_map, get_location(offset + prefix.length));
 	}
 	const processed_with_map = StringWithSourcemap.from_processed(processed.code, decoded_map);
@@ -164,7 +241,7 @@ export default async function preprocess(
 
 	async function preprocess_tag_content(tag_name: 'style' | 'script', preprocessor: Preprocessor) {
 		const get_location = getLocator(source);
-		const tag_regex = tag_name == 'style' 
+		const tag_regex = tag_name === 'style'
 			? /<!--[^]*?-->|<style(\s[^]*?)?(?:>([^]*?)<\/style>|\/>)/gi
 			: /<!--[^]*?-->|<script(\s[^]*?)?(?:>([^]*?)<\/script>|\/>)/gi;
 
diff --git a/test/sourcemaps/samples/source-map-generator/_config.js b/test/sourcemaps/samples/source-map-generator/_config.js
new file mode 100644
index 000000000..fefb776f3
--- /dev/null
+++ b/test/sourcemaps/samples/source-map-generator/_config.js
@@ -0,0 +1,25 @@
+import MagicString from 'magic-string';
+import { SourceMapConsumer, SourceMapGenerator } from 'source-map';
+
+export default {
+	preprocess: {
+		style: async ({ content, filename }) =>  {
+			const src = new MagicString(content);
+			const idx = content.indexOf('baritone');
+			src.overwrite(idx, idx+'baritone'.length, 'bar');
+
+			const map = SourceMapGenerator.fromSourceMap(
+				await new SourceMapConsumer(
+					// sourcemap must be encoded for SourceMapConsumer
+					src.generateMap({
+						source: filename,
+						hires: true,
+						includeContent: false
+					})
+				)
+			);
+
+			return { code: src.toString(), map };
+		}
+	}
+};
diff --git a/test/sourcemaps/samples/source-map-generator/input.svelte b/test/sourcemaps/samples/source-map-generator/input.svelte
new file mode 100644
index 000000000..0d942390f
--- /dev/null
+++ b/test/sourcemaps/samples/source-map-generator/input.svelte
@@ -0,0 +1,12 @@
+<h1>Testing Styles</h1>
+<h2>Testing Styles 2</h2>
+<script>export const b = 2;</script>
+<style>
+	h1 {
+		--baritone: red;
+	}
+
+	h2 {
+		--baz: blue;
+	}
+</style>
diff --git a/test/sourcemaps/samples/source-map-generator/test.js b/test/sourcemaps/samples/source-map-generator/test.js
new file mode 100644
index 000000000..72ad1da1d
--- /dev/null
+++ b/test/sourcemaps/samples/source-map-generator/test.js
@@ -0,0 +1 @@
+export { test } from '../preprocessed-styles/test';