Bumgarner, 30, revealed his alternate identity in a story published Sunday by The Athletic.
The four-time All-Star and three-time World Series champion has been roping for years under the alias Mason Saunders. Bumgarner learned rodeo when he was a teenager. He said other players and teams knew about his hobby as a roper on horseback, but the media didn't until now. There are several pictures of Bumgarner (Saunders) competing on social media.
He told The Athletic he uses a different name while competing so he doesn't draw too much attention to himself. Bumgarner competes part-time now, but said he might try to become a full-time competitor once he retires from baseball.
"No matter what hobbies I have, I take them serious," Bumgarner said. "That's just my personality. I don't do anything just for fun, per se. I wish I did."
Bumgarner won $26,560 during a team rodeo event in December in Wickenburg, Ariz., about two weeks before signing a five-year, $85 million contract with the Diamondbacks. He competed in another event days before pitching for the Giants last year during spring training.
Diamondbacks general manager Mike Hazen told The Athletic he wasn't going to get into talking about specific contract language when asked if Bumgarner is allowed to compete in rodeo events under the terms of his new contract.
Hazen addressed Bumgarner's hobby again Monday in Scottsdale, Ariz. He said he did not know Bumgarner had a history of competing in rodeo events. Hazan also said Bumgarner has not competed in rodeo events since joining the Diamondbacks.
"Madison is a grown man, and we know that he is committed to helping us achieve our goals as a team," Hazen told MLB.com. "Those have been the conversations that we've had from the time we first talked to him until very recently."
Bumgarner had a 3.90 ERA and went 9-9 in a league-high 34 starts last season. The four-time All-Star has a career ERA of 3.13 with a 119-92 record in 289 games during his decorated MLB tenure. The left-handed pitcher -- who ropes right handed -- has also been known for his hitting power during his baseball career. Bumgarner has 19 career home runs in 11 seasons.
The Diamondbacks face Bumgarner's former team -- the San Francisco Giants -- in a spring training game at 3:05 p.m. EST Monday at Scottsdale Stadium in Scottsdale, Ariz.