Bumgarner's May ERA ranked third and his opponents' batting average was the fifth lowest (.203) among the league's qualifying pitchers. He allowed one earned run or fewer in four outings, lowering his overall ERA from 3.74 to 2.85.
Bumgarner also struck out 10 batters in his final two starts of the month, against Minnesota on May 25 and St. Louis on May 30. Those were the first consecutive double-digit strikeout efforts for Bumgarner, 24, since 2011.
"The last two starts I felt really good," Bumgarner said. "... I feel like I'm always concentrating on trying to get better and working on stuff. I'm glad to see it paying off."
Bumgarner probably clinched the award last Friday at St. Louis when he bested Cardinals ace Adam Wainwright. Bumgarner worked seven shutout innings and struck out 10 as the Giants triumphed, 9-4.
According to the Elias Sports Bureau, Bumgarner joined Jason Schmidt (5-0, 54 strikeouts in May 2004), Ray Sadecki (6-0, 48 strikeouts in September 1967) and Juan Marichal (6-0, 60 K's in September 1963) as the only modern-era Giants hurlers to record at least five wins in a month without a loss while fanning at least 48.
Bumgarner helped the Giants post a 19-8 mark in May, a surge that gave them the best record in the Major Leagues.
"I'm thankful for the opportunities to go out there and pitch," he said.
Chris Haft is a reporter for MLB.com. Read his blog, Haft-Baked Ideas, and follow him on Twitter at @sfgiantsbeat. This story was not subject to the approval of Major League Baseball or its clubs.