The download will be available as a free update for users of Windows 8.
The full-blown retail launch with boxed discs will be on the following day, The Verge reported Wednesday.
Microsoft's manufacturing partners are expected to receive the final version of 8.1 by the end of August, giving them time to have it available on their hardware by the date of the launch, PC World reported.
Windows 8.1 is seen as an attempt to mollify desktop diehards who have expressed deep dislike of the new-look modern user interface, geared toward touchscreens and mobile devices.
Windows 8.1 will let them bypass the UI and boot directly to the desktop.
Microsoft has said Windows 8.1 represents a significant update with new features and apps.