diff --git a/.changeset/fix-avatar-zero-size-header.md b/.changeset/fix-avatar-zero-size-header.md
new file mode 100644
index 0000000000000..bf6e90ee53426
--- /dev/null
+++ b/.changeset/fix-avatar-zero-size-header.md
@@ -0,0 +1,5 @@
+---
+"@rocket.chat/meteor": patch
+---
+
+Fix `Content-Length` header being omitted when avatar file size is zero, violating HTTP spec.
diff --git a/apps/meteor/server/routes/avatar/utils.spec.ts b/apps/meteor/server/routes/avatar/utils.spec.ts
index 5c31e286467ab..0b10f3a74a39d 100644
--- a/apps/meteor/server/routes/avatar/utils.spec.ts
+++ b/apps/meteor/server/routes/avatar/utils.spec.ts
@@ -81,6 +81,19 @@ describe('#serveAvatarFile()', () => {
 		expect(response.setHeader.getCall(3).calledWith('Content-Length', file.size)).to.be.true;
 		expect(mocks.fileUploadGet.calledWith(file, request, response, next)).to.be.true;
 	});
+
+	it('should serve avatar and set Content-Length 0 when file size is exactly 0', () => {
+		const zeroSizeFile = { uploadedAt: new Date(0), type: 'image/png', size: 0 };
+		const request = { headers: { 'if-modified-since': new Date(200000).toUTCString() } };
+		serveAvatarFile(zeroSizeFile, request, response, next);
+
+		const contentLengthCall = response.setHeader.getCalls().find(
+			(c) => c.args[0] === 'Content-Length'
+		);
+
+		expect(contentLengthCall).to.exist;
+		expect(contentLengthCall?.args[1]).to.equal(0);
+	});
 });
 
 describe('#serveSvgAvatarInRequestedFormat()', () => {
diff --git a/apps/meteor/server/routes/avatar/utils.ts b/apps/meteor/server/routes/avatar/utils.ts
index 9dbb50719ab13..512ee9f76ba88 100644
--- a/apps/meteor/server/routes/avatar/utils.ts
+++ b/apps/meteor/server/routes/avatar/utils.ts
@@ -40,7 +40,7 @@ export const serveAvatarFile = (file: IUpload, req: IIncomingMessage, res: Serve
 		res.setHeader('Content-Type', file.type);
 	}
 
-	if (file.size) {
+	if (typeof file.size === 'number') {
 		res.setHeader('Content-Length', file.size);
 	}