Hector Sanchez had two RBIs to back Zito (3-1), who won the 2002 AL Cy Young Award across the bay in Oakland but left for a $126 million, seven-year deal with the Giants before the 2007 season.
Josh Donaldson homered, doubled in a run and hit a three-run double off Steve Edlefsen for the A's, giving Donaldson career-bests of three hits and five RBIs. But Oakland couldn't overcome a shaky night by starter Jarrod Parker (1-2) and lost their 10th straight in San Francisco's waterfront ballpark dating to 2009.
Zito matched his season high with five strikeouts in 5 1-3 innings and also drew a bases-loaded walk in the Giants' four-run third inning. The A's walked five that inning as San Francisco batted around on the way to scoring its most runs at home since getting eight last Sept. 12 against the Padres. Zito, who improved to just 2-4 with a 7.16 in six starts against the A's, walked off to a warm ovation from the sellout crowd of 41,477 with one out in the sixth after allowing Jemile Weeks' leadoff triple and an RBI single to Cliff Pennington.
Belt had an RBI single in the second, drew a bases-loaded walk in the third that chased Parker, and hit a sacrifice fly in the fourth.
Zito came in with the lowest ERA of San Francisco's starters at 2.53 but raised it to 3.00 and fell behind Matt Cain's 2.67.
This has been a comeback season for the 34-year-old lefty, who was kept off the roster for all three playoff rounds during the Giants' run to the 2010 World Series title.
Santiago Casilla, San Francisco's fifth reliever, pitched out of a jam in the ninth for his 10th save in 11 chances. He retired Weeks on a groundout with runners on the corners to end it.
Belt put the Giants ahead 1-0 on an RBI single in the second that scored Angel Pagan, who drew a leadoff walk and stole second. Pagan, coming from second, made a headfirst slide into home but easily beat the throw.
A's pitchers went 0 for 18 with seven strikeouts at the plate last season in interleague play and Parker struck out in the lone plate appearance Friday, extending the pitchers' hitting funk to 0 for 22 dating to Ben Sheets' single on June 19, 2010, at St. Louis.
"Oh, I didn't realize that," Oakland manager Bob Melvin said. "Hopefully we'll be able to pick up on that."
San Francisco's longest home winning streak against an opponent at AT&T Park is 12 games, against the Mets from May 1, 2000-May 16, 2003 -- so the Giants could match that with a weekend sweep. They have a nine-game home winning streak against the Texas Rangers, who visit AT&T Park from June 8-10 still seeking their first win here.
A special logo was painted on the grass behind home plate in honor of next week's 75th anniversary of the Golden Gate Bridge, and CEO and team president Larry Baer searched before the game for his Golden Gate Bridge neck tie.
Players wore special patches on their jerseys for the occasion, too.
Joaquin Arias had a sacrifice fly, returning to San Francisco's lineup after missing two games since being hit in the left forearm by a pitch Tuesday.
A's lefty Travis Blackley, claimed off waivers this week from the Giants, entered the ballpark on his old San Francisco ID and pitched a 1-2-3 eighth against his old team.
Notes: Giants C Buster Posey had a planned night off and Sanchez caught Zito as has been customary. ... RHP Brad Penny signed a minor league deal with the Giants. ... A's slugger and CF Yoenis Cespedes was set to have his injured left hand examined by a team doctor to determine whether he can start swinging. ... Melvin said that he appreciated receiving a call of apology from Thursday's home plate umpire, Laz Diaz, who ejected the manager for arguing a call on a suicide squeeze. ... Melvin went with Donaldson at 3B a day after he had an error that nearly cost Oakland its 5-4, 10-inning win at Texas. ... The Giants are in a stretch with 20 straight games before their next day off May 31. ... A Zito No. 75 signed orange jersey was donated for charity in commemoration of the bridge anniversary. ... A's OF Coco Crisp, on the DL with an inner ear infection, did baseball activities and took batting practice. He is expected to do more Saturday and perhaps be activated as soon as Sunday.