Alex Gonzalez and Lyle Overbay added solo drives for the Blue Jays, who have a club-record 53 homers in May and a majors-best 88 overall. Toronto, which also swept a series at Baltimore from April 9-11, recorded consecutive three-game sweeps of the Orioles for the first time since August 2001.
"There isn't a whole lot of margin for error when you're facing a team that's hit as many home runs as these guys have," Orioles manager Dave Trembley said. "They continue to capitalize on mistakes."
Romero used his sinker to get 17 ground-ball outs and gave up just two fly balls.
"He had a good cutter going and a good sinker," Blue Jays catcher John Buck said. "Usually it's just cutter and changeup but he had a good sinker going and was able to get some ground balls when we needed them."
Bautista's lone defensive play was Luke Scott's fly ball to right in the second, Vernon Wells didn't make a single play in center, and the only thing hit to left fielder Fred Lewis was a Julio Lugo fly ball in the eighth.
"Any boring day when they only score one run is a good day," Bautista said.
Romero (5-2) struck out seven and walked two in his second complete game of the season. The left-hander retired 15 of 18 after Nick Markakis' RBI single in the first inning.
"His changeup was good today and he kept it down," Baltimore's Ty Wigginton said. "Any time you're changing speeds and moving the ball around the plate, in and out, and down, that's usually the ingredients for a big game."
It was a marked improvement from Romero's previous start on Tuesday, when he was roughed up by the Los Angeles Angels while pitching in his hometown before some 200 family and friends. He lasted just 5 1/3 innings in that one, allowing seven runs and 11 hits in an 8-3 loss.
"I think I was a little too excited last start," Romero said. "Being at home it was a little bit overwhelming. After I sat down and looked back a little, I was out of character a little bit."
The Orioles have lost five straight and have the worst record in the majors at 15-36. Things don't get any easier for them either.
Baltimore is off Monday before opening a three-game series at Yankee Stadium. After that, the Orioles return home to host Boston and New York.
Outfielder Adam Jones said the one-day break is coming at the right time.
"We've got a good day off tomorrow," Jones said. "It's nice weather. Everybody should go out and enjoy New York, watch a play, get our minds off baseball and come back against the Yankees."
Bautista hit Will Ohman's first pitch of the eighth over the wall in center for his 12th homer in May, matching Jose Cruz Jr. (Aug. 2001) and Carlos Delgado (Aug. 1999) for the most by a Blue Jays player in a single month.
Gonzalez followed with a drive to left, his 11th. It's the sixth time this season Toronto has hit back-to-back homers.
Overbay led off the sixth with his seventh homer and third in two days. Overbay hit a solo drive and a two-run shot in Saturday's 5-2 win.
"They're locked in at the plate, especially the guys in the middle of the lineup," Trembley said. "They're swinging the bats very aggressively."
Toronto got off to a fast start against Jeremy Guthrie (3-5), scoring three runs in the first. Lewis led off with a double and scored on Wells' two-out double. Bautista then walked and Gonzalez and Overbay followed with RBI singles.
"The loss was really dictated by that first inning," Guthrie said. "I take full responsibility on this one."
Guthrie allowed four runs and seven hits in six innings to lose for the first time since May 3 against the Yankees. He had won his last three decisions
Toronto will activate LHP Brian Tallet off the disabled list to start Tuesday against Tampa Bay. Tallet has been out since April 18 with a sore forearm. He'll start in place of LHP Dana Eveland, who was designated for assignment last week. ... Toronto's 53 homers in May are the most by any team in a single month since Atlanta hit 55 in May 2003.