Phelps was suspended from competition by USA Swimming for three months in February after a picture surfaced in a London tabloid showing him inhaling from a marijuana pipe.
It was a low point after winning eight gold medals at the Beijing Olympics.
Tuesday, he marked the occasion like any other day: He woke up late and headed to the pool.
Speaking exclusively with The Associated Press, Phelps said he didn't even realize his suspension ended Tuesday.
Coach Bob Bowman couldn't resist making a joke, saying he planned to enter his star swimmer in a meet later that night.