Jury verdicts in serious criminal cases must be unanimous, the US Supreme Court ruled Monday — striking down state laws that critics say had historically racist roots.

Oregon currently allows jurors to convict such defendants without being unanimous — and Louisiana did up till last year, too — under a system created by racist and anti-Semitic sentiments that aimed to block minority jurors’ opinions, critics have said.

The high court voted 6-3 to ban such non-unanimous verdicts, with Justice Neil Grouch noting the historic discrimination at the heart of them.

“Louisiana first endorsed non-unanimous verdicts for serious crimes at a constitutional convention in 1898,” he wrote. “According to one committee chairman, the avowed purpose of that convention was to ‘establish the supremacy of the white race,’ and the resulting document included many of the trappings of the Jim Crow era.”

Gorsuch added that when the Sixth Amendment’s right to a fair trial was made part of the Constitution in 1791, it was understood that the term included a unanimous jury.

“Whenever we might look to determine what the term ‘trial by an impartial jury trial’ meant at the time … the answer is unmistakable,’’ Gorsuch wrote. “A jury must reach a unanimous verdict to convict.’’

The top court’s ruling is an example of how “originalism’’ — or a concept favored by conservatives, in which the Constitution is interpreted in the context of the Founding Fathers — can sometimes lead to a liberal result.

Conservative Justices Clarence Thomas and Brett Kavanaugh were among those siding with Gorsuch, although for different reasons.

The court’s decision involved the conviction of Evangelisto Ramos, who has been serving a life sentence in Louisiana since being found guilty in a 10-2 vote of killing a woman in 2016.

While last year Louisiana residents voted to ban such non-unanimous jury verdicts going forward, Ramos’s previous conviction was not covered by the change.

Oregon’s state attorney general, Ellen Rosenblum, also supported unanimous juries — although she, too, wanted it to address only cases going forward, not retroactively.

At the heart of the legal issue was a 1972 supreme-court ruling that had upheld the idea of non-unanimous juries for states. Unanimous verdicts are required in federal cases.

Gorsuch, referring to the 1972 split ruling, said, “As judges, it is not our role to reassess whether the right to a unanimous jury is ‘important enough’ to retain.”

He noted that some criminal cases are so minor that they may not require a jury trial, hence the ruling having to only cover serious ones.

Ramos’s lawyer, Ben Cohen, said in a statement, “We are heartened that the court has held, once and for all, that the promise of the sixth amendment fully applies in Louisiana, rejecting any concept of second-class justice.’’

Ramos and others found guilty by non-unanimous juries may now have their convictions tossed and be given new trials — a fear that critics say will overwhelm the legal system.

The three dissenting supreme-court judges were Chief Justice John Roberts, Samuel Alito and Elena Kagan.

Alito, who wrote the dissent, said the decision “imposes a potentially crushing burden on the courts and criminal-justice systems of those states.’’

-With Post Wires