Conor Gillaspie, making a rare start at third base in place of Pablo Sandoval, hit an inside-the-park home run in the seventh while Brandon Crawford added two hits and an RBI for the Giants, who have won two straight following a four-game losing streak.
That's little consolation for the defending World Series champs, who will still miss the postseason a year after claiming their first title in 54 years.
Giants manager Bruce Bochy juggled his lineup slightly, starting Gillaspie at third and moving Sandoval to first. It seemed to do the trick.
Gillaspie singled and scored on Belt's homer in the fourth then stumbled his way around the bases in the seventh for his first career home run.
The Giants rookie hit a deep fly ball into the gap in right center that sailed over the head of Colorado outfielder Ty Wiggington. Gillaspie raced around the bases and broke for home when Wiggington overthrew cutoff man Mark Ellis. He tripped after rounding third then had to hustle to beat the throw home from shortstop Tommy Field.
Belt hit his ninth homer, a two-run shot off Rockies starter Alex White, in the fourth. It was Belt's first splash hit and the 84th overall at the Giants waterfront ballpark.
The only player younger than the 23-year-old Belt to reach the waters at ATA&T Park is Sandoval, who did it 12 days shy of his 23rd birthday. Giants hitters have done it a total of 60 times, 35 from home run king Barry Bonds.
Belt, who spent three stints in the minors this season after starting the year with the big league club, later walked and scored on Crawford's triple in the sixth.
That was all the support Bumgarner needed to secure San Francisco's seventh consecutive win over Colorado and 13th overall this season.
Bumgarner, the winning pitcher in Game 4 of the World Series last season, allowed only two hits and pitched with runners on base in only two innings - the second and sixth. Each time, he worked out of trouble.
Kevin Kouzmanoff doubled leading off the second and took third on Jordan Pacheco's single to right. Bumgarner (13-13) then struck out the next three hitters, including Wilin Rosario and Field who were both caught looking at a third strike.
Bumgarner retired 18 of the final 19 batters he faced, did not walk anyone for the second straight start and finished with nine strikeouts.
Ramon Ramirez pitched the eighth and former starter Barry Zito worked the ninth to complete the shutout.
White (2-4) got off to a rough start, walking leadoff batter Andres Torres on six pitches then uncorking back-to-back wild pitches to advance Torres to third. Mike Fontenot followed with a sacrifice fly to give the Giants a 1-0 lead.
San Francisco put two runners on in the second and had a runner at second in the third but couldn't add on until Gillaspie singled in the fourth and Belt homered.
Crawford's RBI triple was the second of the rookie's career. He later scored on a wild pitch, the fourth of the night thrown by Colorado pitchers.
Notes: Rockies manager Jim Tracy will bring back his entire coaching staff next season, including hitting coach Carney Lansford and pitching coach Bob Apodaca, both of whom have come under fire this year. ... Colorado general manager Dan O'Dowd and slugger Jason Giambi held a lengthy meeting in the dugout two hours before the game discussing the future of the club and some of the players. ... LHP Drew Pomeranz makes his fourth career start for Colorado. Pomeranz (1-1) allowed two runs over 5 2-3 innings but left with a no-decision against the Giants on Sept. 17. ... RHP Matt Cain (12-11) goes for his third straight win over the Rockies in the finale. ... Bill Neukom, who will retire as the Giants' controlling owner at the end of the year and be replaced by Larry Baer as chief executive officer, was on the field during batting practice. ... Oakland Raiders quarterback Jason Campbell and winger Ryan Clowe of the San Jose Sharks were among those in attendance.