Users will be able to turn their phones and tablets into de facto portable PC monitors for playing Steam games around the house later this month. That's when Valve has announced it will be launching its new Steam Link app for iOS and Android, officially bringing Steam's long-established in-home streaming feature to mobile devices for the first time (though there have been unofficial apps to provide similar functions for a while now).

The free Steam Link app will be available for phones and tablets running Android or iOS (though Android support will be in beta for launch) starting the week of May 21. Apple TV and Android-based TV platforms will also run the software, Valve said, essentially turning those set-top boxes into versions of Valve's own standalone Steam Link hardware

The mobile app will link to a "Mac or PC" host system, according to the announcement (Update: A Valve representative tells Ars Linux and SteamOS are also supported), relaying game images and sound and transmitting controller input over 5Ghz Wi-Fi or wired Ethernet. While Valve said the app will include support for the Steam Controller, MFi-certified iOS controllers, "and more," it's unclear whether more generic USB/bluetooth controllers and/or mouse/keyboard setups will be usable via the mobile app (Update: A Valve representative tells Ars that keyboard/mouse controls will be supported, with a fuller list of compatible controllers available before launch).

Valve's announcement comes as more and more major game developers are releasing full-fledged ports of their titles on mobile platforms , targeting the wider market of smartphone owners who enjoy the freedom of playing away from a TV or monitor. Mobile gaming now represents a slim majority of all worldwide spending on video games , according to a recent NewZoo report.

Valve also announced an accompanying Steam Video app, available this summer, that will let users watch "thousands of movies and shows" available on Steam directly via a Wi-Fi or LTE connection. That could be an important new viewing option if Valve wants its more gaming-focused Steam service to compete on non-interactive media with the likes of iTunes, Google Play, etc.