Do You Have to Be Online to Play a Game on Steam?by Rusty CaulfieldUpdated September 22, 2017
Steam, a video-game, digital-distribution system launched in 2003, mostly requires that you use your Internet connection to play its games. However, if you are temporarily without an Internet connection, you can use Steam’s Offline Mode with some products.
Offline Mode is primarily useful for single-player games that don’t require an Internet connection to play. Your Steam application must be properly set up in order to use it.
Activating Offline Mode
Setting Steam to run in Offline Mode can be done by clicking on “Steam” from the main menu and selecting “Go Offline.” Then click “Restart in Offline Mode” to cause Steam to restart and allow you to play games offline.
To play a game in Offline Mode, both Steam's application files and the individual game files must be up to date. If necessary, run Steam in Online Mode to update these files first.