Judge Matthew Brann said the suit, which rested on allegations of irregularities, was "without merit".
The move paves the way for Pennsylvania to next week certify Joe Biden's win - he leads by more than 80,000 votes.
It is the latest blow to Donald Trump, who is trying to overturn his loss in the 3 November presidential election.
He has refused to concede and made allegations of widespread electoral fraud, without providing any evidence.
The lack of a concession has upended the process that normally follows a US election.
Mr Biden is projected to defeat President Trump 306 to 232 in the US electoral college, which determines who becomes president - far above the 270 he needs to win.
The Trump campaign has lost a slew of lawsuits contesting results from the election, and its latest efforts focus on stopping the swing states that handed Mr Biden his win certifying the results - an essential step for the Democrat to be formally declared victor.