Neptune was the codename for a version of the Windows operating system that was under development by Microsoft during 1999. It was based on Windows 2000 and was scheduled to be the first consumer version of Windows to be built on NT code, to replace the Windows 9x series.

In early 2000, Microsoft merged the team working on Neptune with that developing Windows Odyssey, the upgrade to Windows 2000 for business customers. The combined team worked on a new project codenamed Whistler, which was released at the end of 2001 as Windows XP. In the meantime, Microsoft released another home user DOS-based operating system called Windows Me.

Only one alpha build of Neptune, 5111, was released to testers under a non-disclosure agreement, and later made its way to various beta collectors' sites and virtual museums. Also, there were at least two other builds, which were Build 5000, which was released to very few people, and Build 5022.^{[citation needed]} Build 5000 is very heavily debated.^{[citation needed]} Files marked 5022 are in the Windows Me beta.^{[citation needed]}