The measure, which was passed by Republican legislators and signed into law by Gov. Tom Corbett in March 2012, requires voters to present an acceptable form of identification when they show up at polls, the Pittsburgh Post-Gazette reported.
Commonwealth Court Judge Bernard McGinley said the law "unreasonably burdens the right to vote" and posed "a substantial threat" to hundreds of thousands of qualified voters.
"Voting laws are designed to assure a free and fair election; the Voter ID Law does not further this goal," McGinley said in his ruling.
State Democrats said they were pleased the law was struck down.
"Senate Democrats have said clearly and repeatedly that the voter ID law was an overreach that would result in the disenfranchisement of hundreds of thousands of voters," state Senate Minority Leader Jay Costa, D-Forest Hills, said in a written statement. "It was a law that should have never been approved and we are very happy that the court turned aside the measure today."
Witold Walczak, the legal director of the American Civil Liberties Union of Pennsylvania who lead attorneys on the challenge of the law, said: "Once the Commonwealth admitted they couldn't identify any of the fraud supposedly prevented by the voter ID law, the act was plainly revealed to be nothing more than a voter suppression tool."
The governor's office has not yet commented on McGinley's ruling.