A violent prisoner left uncuffed and unguarded in a Brooklyn hospital bashed a worker who was taking care of him, a lawsuit charges.

Justin Houston — who was hauled to Brookdale Hospital by cops — lay in wait for Roxanne Sinclair, hiding his unbound arms under a blanket, the 50-year-old nurse’s assistant alleges.

The cop who was supposed to be guarding Houston was at the nurses’ station when the prisoner pounced, punching the Canarsie resident.

“The next thing I know, I’m on a bed,” Sinclair said. “They [other nurses] told me there was blood on the floor and my clothes were full of blood.”

A spokeswoman for the city’s Law Department it was reviewing the suit.