Stallman is the founder of the Free Software Foundation and initiated development of the Unix-like GNU operating system.
In a blog post today, he expressed concerns over Valve bringing its Steam client to Linux and potentially other free operating systems.
"I suppose that availability of popular non-free programs on GNU/Linux can boost adoption of the system," Stallman said.
"However, our goal goes beyond making this system a 'success'; its purpose is to bring freedom to the users. Thus, the question is how this development affects users' freedom."
'Free' in this case has nothing to do with monetary cost, but instead refers to a user's ability to alter and reproduce code.
Steam employs digital rights management (DRM) to protect its games against piracy; however, the practice can also limit a user's ability to alter, modify or reproduce code from the games.
"Non-free game programs (like other non-free programs) are unethical because they deny freedom to their users. (Game art is a different issue, because it isn't software.) If you want freedom, one requisite for it is not having nonfree programs on your computer.
"However, if you're going to use these games, you're better off using them on GNU/Linux rather than on Microsoft Windows. At least you avoid the harm to your freedom that Windows would do," he continues.
"Thus, in direct practical terms, this development can do both harm and good. It might encourage GNU/Linux users to install these games, and it might encourage users of the games to replace Windows with GNU/Linux. My guess is that the direct good effect will be bigger than the direct harm."
Despite the potential positive effect Steam could have on Linux, Stallman is still cautious in his support - the primary concern being that the availability of non-free software on the operating system will alter the development community and dilute the purpose of open-source software.
Stallman's criticisms come a week after Valve head Gabe Newell reiterated the company's plans to bring Steam to Linux, during which he calling Windows 8 a "catastrophe" in the process.