A Long Island psychiatric patient escaped from an ambulance that was transporting her between two medical centers Friday night, cops said.

Marlena Berretta, 29, jumped from the ambulance as it idled at a stoplight on New Moriches Road near Route 347 in Lake Grove about 5:50 p.m., police said.

The patient was being transferred from Stony Brook University Hospital Comprehensive Psychiatric Emergency Program to South Oaks Hospital in Amityville when she fled, authorities said.

She was last seen heading eastbound on Route 347, a busy stretch of state highway with up to three lanes of traffic in each direction.

Police said Berretta is currently homeless.