The South Troendelag District Court in western Norway on Thursday also banned Northug from driving for life, which normally means a minimum of five years, after crashing his car in May.
Northug said he will regret the incident for the rest of his life and told the court he accepted its ruling.
"I have had some dark days," Northug said. "This will follow me throughout my whole life."
A date for his prison sentence had not been set. Northug said he will not appeal, but since the sentence is shorter than four months he can request to serve it as house arrest.
It was not immediately clear if Northug would be allowed to compete again for his country although the Norwegian Ski Association said it would not penalize Northug because "the events of May 4 had nothing to do with competition or training."
"It's not in sports regulations to punish him from our side in any way," association president Erik Roeste told Norwegian news agency NTB, but declined further comment.
The International Ski Federation FIS declined to comment on the case but said that any restrictions or conditions set on skiers in similar cases would not come from FIS but from national organizations.
The incident and its aftermath has been front-page news in the winter-sport crazy nation, where Northug is a massive star and celebrity.
Northug had been driving, with a friend in the passenger seat, when he crashed into a barrier on a Norwegian highway on the night of May 4.
Initially the friend agreed to take the blame for the accident. Northug went home and told police who had come to his house that he wasn't sure who had driven the car.
Later, he admitted he had been the driver but blamed his earlier denials on a "very mixed state of mind."
"It was a combination of having alcohol in my blood and the fact that I had just woken up," he told reporters in Trondheim after the court adjourned.
Northug apologized for his crime and behaviour, saying he hadn't been thinking clearly on that day and deeply regretted driving under the influence of alcohol. His friend sustained minor injuries in the crash, but no one else was hurt.
His friend sustained minor injuries in the crash, but no one else was hurt.