Michael Parrish, a businessman and former Army aviator from Malvern, is already in the race.
Trivedi, a physician who lives in Birdsboro, was a battalion surgeon in the Navy. He ran against Gerlach in 2010 and 2012, losing each time by about 14 percentage points.
"An open seat is a much different race," Trivedi said Monday. "I think we'll get a much fairer shot, a much more level playing field."
Parrish, in a statement, said he looked forward to "a spirited discussion."
"I have confidence," he said, "that voters in the primary will select the strongest nominee to ensure a Democrat wins this very tough district in November."
Parrish is a former Republican. His allies paint his party switch as a measure of his independence in a moderate district.
Trivedi has yet to begin raising money for his campaign, but is touting a list of local endorsements.
On the Republican side, Chester County Commissioner Ryan Costello is the only candidate. He said he was listening to residents and focusing on the economy. "Who my opponent is isn't my focus," he said.
The open seat in a district with a moderate electorate makes this race one of the country's most taut, at least at the starting line.
Trivedi hopes to focus on jobs. He called for more investment in infrastructure and education.
Parrish similarly touted the importance of education and the economy when he opened his campaign in January.
Republicans painted the Democratic contenders as one (Trivedi) who has twice lost by double digits, and one (Parrish) who "became a Democrat solely to run for Congress."
Gerlach, 58, announced in January that he would not seek a seventh term.