The update was spotted by SamMobile running on the Google Edition of the Samsung Galaxy S4, which is currently only available in the US.
The test build is flagged as version JWR66N.S005.130625. The JWR66N part of the build name refers to the Android version, while S005.130625 is Samsung's own build tracking code.
Android 4.3 has been spotted running on the HTC One and Nexus 4 so far, but it may not be too long until the update is rolled out officially.
- HTC Nexus Edition getting Android 4.3 update in July