"We're seeing people at home, at schools, at small businesses, at large companies, and other organizations adopt Windows 10 faster than ever, and use Windows 10 more than ever before," Yusuf Mehdi, corporate vice president of Microsoft's Windows and Devices Group, said in a blog post.
The adoption speed has likely been ramped up by Microsoft's move to make Windows 10 available as a free upgrade through July 29.
After that, it will be available pre-installed on new devices or for purchase separately, with a home computer version priced at $119, according to the Redmond, Washington-based company.
The tech titan is hoping the new version launched after a weak response to Windows 8 and aimed at integrating with a variety of devices can help it gain traction in the mobile sector, where it lags behind Google Android and Apple iOS, and in emerging technologies for computing.
Although Microsoft dominates the personal computer software market, Windows runs on only a tiny fraction of smartphones.
The 300 million represents new computers and devices with Windows 10 installed, as well as upgrades of existing gadgets.
Microsoft has set a goal of having the operating system in use in more than a billion devices by 2018, which would put it on par with Apple, which said this year it has one billion active devices running iOS.
Microsoft said it would introduce an "anniversary update" of the operating system to make it a more powerful educational tool in classrooms.