The senator had not been expected to retire and will likely cruise to reelection. He hasn't faced a tough race since he first won the seat in 1994.
Inhofe said he made his decision shortly after President Obama won reelection.
"I went home and I talked to my wife, and I said you know we've got a serious problem here. We’re going to have this guy around for four more years. I just can't bail out now."