Quote Originally Posted by GameSpot
"We are not going to support an offline mode," Del Priore replied. "But as I said before, nothing in DIablo 4 is going to require partying up. You can play solo and dungeons are private. Campain quest areas will [also] remain private."

This shouldn't be much of a surprise to Diablo players on PC, especially those that played Diablo 3, which has required an internet connection to function since its launch. This is also the case for the majority of Blizzard's other major franchises, such as World of Warcraft, Overwatch, and StarCraft.

However, console versions of Diablo 3 do include the ability to play offline. Diablo 4 is in development for PS4 and Xbox One, as well as PC. In a statement to GameSpot, a Blizzard representative has confirmed that Diablo 4 will be online-only across all versions, including PS4 and Xbox One.
Diablo 4 Won't Have An Offline Mode, All Versions Are Online-Only - GameSpot