The justices voted 4-3 to send the plan back to the Legislative Reapportionment Commission, and the majority said their opinion in the case, laying out the reasoning, would be released later.
The high court's ruling immediately threw into disarray plans by candidates and parties for this year's General Assembly races.
The two-page order said current district lines will remain in force until the commission comes up with a new plan that passes legal muster.
The commission consists of the Republican and Democratic floor leaders from the House and Senate, along with a fifth member, an appointed judge.
The plan the court threw out was opposed by Senate Democrats and others, who argued in a court session Monday that there was not sufficient reason to split some counties and towns, and that decisions were overly driven by political considerations.
The one-paragraph dissenting statement, written by Justice Thomas Saylor and joined by justices J. Michael Eakin and Joan Orie Melvin, said they were not persuaded the plan was contrary to law "as reflected in the existing precedent."
"Although I am receptive to the concern that past decisions of the court may suggest an unnecessarily stringent approach to equalization of population as between voting districts, I believe this could be addressed via prospective guidance from the court," Saylor wrote.
The plan the commission developed moved some House and Senate districts across the state, and Senate Democrats argued they were able to produce a map that divided far fewer municipalities and counties.
Part of the court order changed certain deadlines for the primary petition process for candidates, and the majority said any signatures on nominating petitions that were collected on Tuesday and Wednesday, the first two days of the three-week petition circulation period, would be considered valid.