diff --git a/integration/example/build-node-modules.civet b/integration/example/build-node-modules.civet
new file mode 100644
index 000000000..f2dfeff75
--- /dev/null
+++ b/integration/example/build-node-modules.civet
@@ -0,0 +1,14 @@
+// This file is used in test/integration.civet to test sourcemap source for node_modules
+esbuild from esbuild
+
+civetPlugin from ../../dist/unplugin/esbuild.mjs
+
+esbuild.build {
+  +sourcemap
+  entryPoints: ['source/uses-pkg.civet'],
+  outdir: 'dist',
+  bundle: true,
+  plugins: [
+    civetPlugin({})
+  ]
+}
diff --git a/integration/example/source/uses-pkg.civet b/integration/example/source/uses-pkg.civet
new file mode 100644
index 000000000..79b7031c6
--- /dev/null
+++ b/integration/example/source/uses-pkg.civet
@@ -0,0 +1,3 @@
+// This file is used in test/integration.civet to test sourcemap source for node_modules
+import { add } from 'test-civet-pkg'
+console.log add(1, 2)
diff --git a/source/unplugin/unplugin.civet b/source/unplugin/unplugin.civet
index 2e1d49c43..da2c4971b 100644
--- a/source/unplugin/unplugin.civet
+++ b/source/unplugin/unplugin.civet
@@ -545,12 +545,21 @@ export const rawPlugin: Parameters<typeof createUnplugin<PluginOptions>>[0] =
           fsMap.set slashed, compiledTS
           sourceMaps.set slashed, civetSourceMap
 
+      // When id ends with outExt, it was resolved by our resolveId hook as a virtual
+      // file in a plugin namespace; the bundler uses source paths as-is. When id
+      // doesn't end with outExt (e.g., node_modules files resolved natively by the
+      // bundler), the bundler resolves source paths relative to the input file's
+      // directory, so we make the source path relative to that directory.
+      srcFileName := if id.endsWith outExt
+        path.relative outDir, filename
+      else
+        path.relative path.dirname(filename), filename
       jsonSourceMap := sourceMap and
         if sourceMap <? "string"
           JSON.parse(sourceMap)
         else
           sourceMap.json
-            path.relative outDir, filename
+            srcFileName
             path.relative outDir, id
 
       transformed: TransformResult .=
diff --git a/test/integration.civet b/test/integration.civet
index 0cf731344..83a3269e2 100644
--- a/test/integration.civet
+++ b/test/integration.civet
@@ -51,6 +51,23 @@ describe "integration", ->
 
     assert.equal data.sources[0].replace(/\\/g, '/'), "../source/main.civet"
 
+  it "should have correct source map source for civet files in node_modules", ->
+    // Create a minimal test package fixture in node_modules
+    pkgDir := "integration/example/node_modules/test-civet-pkg"
+    fs.mkdirSync pkgDir, {recursive: true}
+    fs.writeFileSync `${pkgDir}/package.json`,
+      JSON.stringify {name: "test-civet-pkg", version: "1.0.0", main: "index.civet"}
+    fs.writeFileSync `${pkgDir}/index.civet`,
+      "export add := (a: number, b: number): number -> a + b\n"
+
+    await execCmd 'bash -c "(cd integration/example && ../../dist/civet --no-config build-node-modules.civet)"'
+    data := JSON.parse(fs.readFileSync("integration/example/dist/uses-pkg.js.map", "utf8"))
+
+    pkgSource := data.sources.find (s: string) => s.includes 'test-civet-pkg'
+    assert.ok pkgSource, "should have a source entry for the test-civet-pkg node_modules file"
+    // Source path should NOT have a doubled node_modules segment
+    assert.equal pkgSource.replace(/\\/g, '/'), "../node_modules/test-civet-pkg/index.civet"
+
   it "should load bundled unplugin when localStorage lacks getItem", ->
     await execCmd '''node -e "globalThis.localStorage = {}; require('./dist/unplugin/unplugin.js')"'''