A brazen crook stole a $400,000 Bentley sports car from a downtown garage — after a bonehead parking attendant left the keys inside, cops said.

The car was allegedly taken by John Agaman, 32, of Jersey City, on Nov. 13 from a Quik Park Garage at 70 Little West 12th St., sources said yesterday.

The car’s owner, college student Bo Li, a China native, was shocked to learn his beloved 200-mph Bentley Supersports Coupe was gone.

“I just go to car park and they told me that the car disappeared. I felt very upset,” said Li, who got the pricy ride from his family, which is in the property-development business in Beijing.

Cops arrested Agaman this week and recovered the Bentley, which had $7,500 in damage to its bumper. The suspect’s landlord said he moved out of his apartment about two months before the alleged theft because he couldn’t pay his rent.