Deonte Thompson returned a kickoff 108 yards for a score after Romo threw a touchdown pass in his preseason debut for Dallas, Flacco had a scoring toss and the Ravens beat the Cowboys 37-30 on Saturday night.
The Ravens (2-0) also got a 26-yard fumble return for a touchdown when Courtney Upshaw picked up a botched handoff from Romo and stayed on his feet while Romo tried to drag him down by his jersey.
Romo's final play was a 31-yard touchdown pass to Dez Bryant midway through the first quarter.
"He looked good," Bryant said. "He wanted to show the world he is '9' and he's a baller."
Dallas is 0-2 in the preseason.
Baltimore led 14-7 before Flacco took the field for the first time late in the first quarter. He threw a 19-yard scoring pass to Torrey Smith for a 24-10 lead late in the first half.
Romo was expected to take about a dozen snaps in first game since surgery for a herniated disk that kept him out of a loss to Philadelphia in playoffs-or-bust finale last December. That ended up being two possessions after a botched exchange with running back DeMarco Murray led to Upshaw's score.
"My mindset was to pick it up quickly," Upshaw said. "Romo was kind of trying to get out of the way, so I just took advantage and broke through his tackle to score a touchdown."
Murray had 25 of his 34 yards rushing and a 21-yard catch on the next drive, which ended with Bryant leaping over Ravens cornerback Dominique Franks at the goal line and falling into the end zone.
Romo was 4 of 5 for 80 yards, and Bryant had three catches for 59 yards after both skipped the preseason-opening loss at San Diego, along with several other offensive and defensive starters.
"Some things came up that were a little different we hadn't seen," Romo said. "We were able to communicate and be on the same page and the receivers, me, the linemen. It just shows we are doing some going things and are further along than you might usually be this early."
Thompson answered Romo's touchdown by getting to the left sideline untouched and having only kicker Dan Bailey to beat, which he did easily around midfield to put the Ravens ahead for good. Thompson also had a 50-yard kickoff return.
Flacco had a rougher start than in Baltimore's win over San Francisco in the exhibition opener, when he led an 80-yard scoring drive on his only possession. But he finished 9 of 14 for 133 yards and is 4-for-4 in getting points with first-year offensive coordinator Gary Kubiak's version of the West Coast offense.
After a drive to the first of three field goals from Justin Tucker, Flacco's only touchdown drive was kept alive by a roughing-the-passer penalty against George Selvie on a third-down incompletion. The Ravens scored two plays later after Flacco's longest pass of the night -- a 38-yard to Jacoby Jones -- and Smith's over-the-shoulder catch in the end zone despite tight coverage from rookie cornerback Terrance Mitchell.
Like Flacco, Romo backup Brandon Weeden wasn't as smooth in his second preseason game. He underthrew Terrance Williams just enough to give Baltimore's Chykie Brown a chance to break up what otherwise would have been a scoring pass and finished 10 of 19 for 129 yards with a sack and an interception.
The Ravens gave up 184 yards and 11 first downs with their first team on the field in the first quarter, including Romo's 83-yard scoring drive. But the backups were solid for a team looking for depth on a unit struggling with injuries.
Ravens RB Ray Rice played only the second series and had two carries for 21 yards, including an 18-yarder. He has a shoulder bruise that isn't considered serious. ... Baltimore CB Jimmy Smith left the game early with a chest contusion that is considered mild. ... Cowboys LB DeVonte Holloman was sidelined by a neck injury. ... Cowboys S Ahmad Dixon was benched because he was late to a walkthrough practice. He was the defensive standout for Dallas against San Diego.