The high court, in a 6-3 ruling Friday, denied a request by Gov. Jerry Brown for a stay, the Los Angeles Times reported. Federal courts have ordered the state to get the number of inmates down to 112,164 -- a drop of about 9,600.
The six justices in the majority gave no reason for their decision.
Justice Antonin Scalia said the order exceeds the power of the federal courts.
"California must now release upon the public nearly 10,000 inmates convicted of serious crimes -- about 1,000 for every city larger than Santa Ana," Scalia wrote.
Brown released a brief statement saying California will persist in appealing to the Supreme Court to have the merits of its case considered.
Corrections officials are considering ways to reduce the prison population, the Times said. They say about 900 inmates suffer from serious illness and are not considered dangerous, and they are working on plans to grant early release to some inmates approaching the ends of their terms.
The state has signed contracts to transfer inmates to private facilities outside California, an alternative that would be costly.