[UPDATE] Konami is aware of the issue and a fix is on the way.
The original story is below.
The PC version of Metal Gear Rising: Revengeance launched today, but its arrival was not without issue.
Numerous Steam users are reporting today on the game's forums and to Kotaku that the single-player game will not function without an Internet connection. The game reportedly will not start while in Steam's offline mode. What's more, if your Internet connection drops, you'll supposedly be kicked from the game right away.
It is possible that the issue is a simple bug and not any kind of DRM from Konami, but that remains to be seen. We've reached out to Konami for comment but have not heard back yet.
As Kotaku points out, the Steam product page for Metal Gear Rising: Revengeance does not inform purchasers that the game requires an Internet connection. The game originally launched for Xbox 360 and PlayStation 3 in February 2013.
The PC version includes previously released DLC (Blade Wolf, Jetstream, and VR Missions) and runs at 60 fps.