diff --git a/src/index.js b/src/index.js
index fcce588..4d3290f 100644
--- a/src/index.js
+++ b/src/index.js
@@ -33,6 +33,11 @@ const toReplacementExpression = (value, name) => {
 		return [JSON.stringify(matches[2]), name];
 	}
 
+	// --define @assign=Object.assign replaces expressions with expressions:
+	if (name[0] === '@') {
+		return [value, name.substring(1)];
+	}
+
 	// --define A=1,B=true produces int/boolean literal:
 	if (/^(true|false|\d+)$/i.test(value)) {
 		return [value, name];
diff --git a/test/__snapshots__/index.test.js.snap b/test/__snapshots__/index.test.js.snap
index 8d8f6a0..fe31181 100644
--- a/test/__snapshots__/index.test.js.snap
+++ b/test/__snapshots__/index.test.js.snap
@@ -1469,6 +1469,30 @@ exports[`fixtures build define with microbundle 5`] = `
 "
 `;
 
+exports[`fixtures build define-expression with microbundle 1`] = `
+"Used script: microbundle -f esm --no-sourcemap --define @Object.freeze=Object
+
+Directory tree:
+
+define-expression
+  dist
+    define-expression.js
+  index.js
+  package.json
+
+
+Build \\"defineExpression\\" to dist:
+56 B: define-expression.js.gz
+40 B: define-expression.js.br"
+`;
+
+exports[`fixtures build define-expression with microbundle 2`] = `1`;
+
+exports[`fixtures build define-expression with microbundle 3`] = `
+"console.log(Object({a:\\"b\\",c:\\"d\\"}));
+"
+`;
+
 exports[`fixtures build esnext-ts with microbundle 1`] = `
 "Used script: microbundle --raw
 
diff --git a/test/fixtures/define-expression/index.js b/test/fixtures/define-expression/index.js
new file mode 100644
index 0000000..6387670
--- /dev/null
+++ b/test/fixtures/define-expression/index.js
@@ -0,0 +1 @@
+console.log(Object.freeze({ a: 'b', c: 'd' }));
diff --git a/test/fixtures/define-expression/package.json b/test/fixtures/define-expression/package.json
new file mode 100644
index 0000000..d540b39
--- /dev/null
+++ b/test/fixtures/define-expression/package.json
@@ -0,0 +1,6 @@
+{
+  "name": "define-expression",
+  "scripts": {
+    "build": "microbundle -f esm --no-sourcemap --define @Object.freeze=Object"
+  }
+}
diff --git a/test/index.test.js b/test/index.test.js
index ad6baab..dc4c428 100644
--- a/test/index.test.js
+++ b/test/index.test.js
@@ -8,6 +8,8 @@ const FIXTURES_DIR = `${__dirname}/fixtures`;
 const DEFAULT_SCRIPT = 'microbundle';
 const TEST_TIMEOUT = 11000;
 
+const sleep = ms => new Promise(r => setTimeout(r, ms));
+
 const join = (arr, delimiter = '') => arr.join(delimiter);
 
 const printTree = (nodes, indentLevel = 0) => {
@@ -39,8 +41,12 @@ describe('fixtures', () => {
 				fixturePath = resolve(fixturePath, fixtureDir.replace('-with-cwd', ''));
 			}
 
+			await sleep(1);
+
 			const output = await buildDirectory(fixtureDir);
 
+			await sleep(1);
+
 			const printedDir = printTree([dirTree(fixturePath)]);
 
 			expect(