Valve claims SteamOS, which is designed for living room PCs, offers "significant performance increases" in graphics processing.
The beta version of the operating system is described as "not yet a finished product", and Valve stresses that it's not appropriate for "non-technical" users.
Importantly, the current release only supports a certain set of hardware. See Valve's Steam OS FAQ for more details.
"SteamOS is definitely not a finished product ready for a non-technical user," the company said on its website.
"That being said, we already use it in our living rooms. We are excited about what it is and what it will become. And the more of you do the same and tell us about your experiences, the quicker those rough edges will be sanded off.
"Users should not consider SteamOS as a replacement for their desktop operating system. SteamOS is being designed and optimised for the living room experience."
300 specially selected beta participants have now received both test Steam Machine hardware and a Steam Controller.
Steam Machines represent Valve's first major drive in the hardware market, and a move it hopes will establish its Steam PC gaming platform in the living room.
The first retail units will ship at an unspecified date in 2014 with multiple configurations offered.
Valve, which is not a hardware manufacturer by trade, has partnered with other computer assembly groups to build a diverse range of living-room PCs. Numerous configurations of the Steam console will be announced over the coming months.