Linux is a popular open-source (a.k.a. freely available) operating system that many companies run in their data centers. It competes vigorously with Microsoft Windows Server.

Microsoft (MSFT) executive vice president Scott Guthrie announced the news on Wednesday morning at the Microsoft Connect conference for software developers in New York. Guthrie was joined on stage by Linux Foundation president Jim Zemlin.

The company also said that Google (GOOG), a rival in cloud and applications, is joining the steering committee of the independent .NET Foundation which promotes the use of Microsoft’s .NET software development technologies. Other members include Samsung, Red Hat, and Microsoft itself. In September, Google announced tools to enable .NET developers to work with the Google Cloud Platform.

The fact that Microsoft is officially joining the Linux Foundation (as a highest tier Platinum member) is a big deal historically, says Holger Mueller, an analyst with Constellation Research. “Windows and Linux were archenemies in the Ballmer era,” he tells Fortune. “Now Microsoft runs Linux everywhere. The question is what took them so long.”

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The news is not as shocking as it would have been a few years ago. Steve Ballmer was Microsoft’s chief executive until two years ago. Current CEO Satya Nadella was named to replace him, and he has demonstrated a more open approach toward non-Microsoft technologies. Furthermore, Microsoft joined the Eclipse Foundation, another key open source organization, last March.

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Microsoft is aware many corporate and other software developers love to work with Linux and want those workloads to run on the Microsoft Azure cloud. Microsoft has also forged an alliance with Red Hat (RHT), which offers the flavor of Linux that most big companies run.

Note: This story was updated to add details on Google joining the .NET effort.