The number, announced by executive Terry Myerson at Microsoft's annual gathering for developers in San Francisco, represents a solid start for Windows 10 after the tepid reaction to its predecessor, Windows 8.
Getting its Windows strategy right is a key part of Microsoft's plans to stay relevant in a mobile-centric world where rivals like Amazon Inc, Apple Inc and Alphabet Inc's Google carry more clout with many consumers and developers.
One reason for the rapid take-up is that Windows 10, released in July, is free for individual consumers who download it during its first year. Enterprise customers must pay for Windows 10.
An anniversary upgrade to be released this summer will also be free for users already running Windows 10, Myerson said on Wednesday.
The giveaway is effectively an admission by Microsoft that few individuals are willing to pay for software, and represents a shift to a strategy embraced by most tech startups of attracting as many users as possible and nailing down how to make money later.
Microsoft laid out its business case last year: the more consumer devices run on Windows, the more potential targets it gains for advertising sales and for its own paid-for applications and services.
Myerson said before launch he was aiming for 1 billion devices running Windows 10 within a few years.
On Wednesday, he did not give a breakdown on the type of devices Windows 10 was being used on. Microsoft still dominates the market for personal computer software, but its share of smartphones - where many people now do the bulk of their computing - is tiny.
Technology research firm IDC is forecasting that smartphones running Windows will account for only 1.6 percent of the global market this year, compared to 83 percent running Google's Android system and 15 percent running Apple's iOS.
Microsoft's main problem is that the range of apps that work on Windows mobile devices lags behind those on Android and iOS, making them less attractive to buyers. The less popular they are, the less developers are motivated to make apps for Windows devices, reinforcing the vicious cycle.
During the developer conference, which Microsoft calls Build, the company also gave an update on its HoloLens augmented-reality technology, which ships to developers on Wednesday.
© Thomson Reuters 2016