The 87-page majority opinion, among the four opinions issued Friday, said state House and Senate districts have to take into account "contiguity, compactness and the integrity of political subdivisions," along with having similar population sizes.
The majority opinion by Chief Justice Ronald Castille was joined fully by the three Democrats on the seven-member court.
Castille's opinion acknowledged that the decision to throw out the plan has created uncertainty in the election season, which is now under way, with candidates circulating nominating petitions for the April 24 primary. Castille said the commission delayed in issuing its plan, which it approved 4-1 in mid-December.
Republican justices Thomas Saylor and J. Michael Eakin filed concurring and dissenting opinions, while Republican Justice Joan Orie Melvin wrote a dissent.
Melvin, whose sister holds a state Senate seat in Allegheny County, said "it is clear that there is no perfect plan" and called the majority's solution "both unprecedented and unnecessary."