Dean will face Republican Dan David, founder of an equities markets research firm called GeoInvesting, in November's general election. David was uncontested in the GOP primary.
Prognosticators consider the seat in the Fourth Congressional District a safe Democratic pickup. Hillary Clinton would have carried the newly drawn district — imposed by the state Supreme Court in February — by nearly 20 percentage points over Donald Trump in 2016.
Dean, 58, of Abington, had the financial support of organized labor and Emily's List, a group that works to elect Democratic women who favor abortion rights. She defeated former congressman Joe Hoeffel and gun-control advocate Shira Goodman.
"No wonder Congress is a mess," she said in a television ad during the primary campaign.
Hoeffel, a former three-term congressman, had argued that his experience would help Democrats fight President Trump.
Some Montgomery County Democrats had hoped Hoeffel would drop out of the race, saying he had run for office time and again but wasn't an effective legislator. He disputed that charge and said healthy competition would make the Democratic nominee a stronger candidate.