A naked motorist has been arrested for masturbating — with a toy gun lodged in his backside — while driving in the US.

Robert Casey was allegedly driving his Jeep Cherokee in St Lucie County when a tow truck driver noticed he was naked and masturbating, according to a report from the Fort Pierce Police Department.

"The male's hands were in his groin area moving around," the report read.

When the 49-year-old was pulled over on Interstate 95, cops said it took him a moment to stop since he was still attempting to get dressed.

When a police officer asked the driver why he was naked, "Casey stated that he has problems with this and he is getting therapy," the report said.

The cop then patted Casey down and found a toy pistol tied to his leg, part of which was hidden in his backside. Another portion of the contraption was tied around his genitals, according to the report.

Casey was arrested on a charge of lewd and lascivious behaviour.