The app is a free download that enables users to access their friends list, chat, browse the Steam marketplace and purchase games.
It is currently in beta, which requires that users opt in to the beta through their Steam account before accessing the app on iOS or Android.
Games cannot actually be played through the app, and Valve has not stated plans to begin offering mobile games through its marketplace. Distribution of games on iOS is thought to be particularly unlikely, with Apple maintaining tight control through its app store.
A Steam mobile app was rumoured last year, as Valve experienced an increase in users requesting an app.
Steam had its seventh consecutive year of over 100% growth in 2011. Growth was attributed to a number of high selling titles like Elder Scrolls V: Skyrim, which became the fastest selling game in Steam's history.
Watch a trailer for Elder Scrolls V: Skyrim below: