"The St. Louis PD tells TMZ ... 25-year-old Maroney was arrested for possession of marijuana and unlawful use of a weapon.

Cops say while Maroney has a valid "Conceal and Carry" permit -- he was arrested for packing the gun while under the influence of drugs.

Maroney was in a car with 5 other people late Monday night when cops say they observed "several occupants" of the vehicle smoking what was believed to be marijuana.

Five of the six people in the car were arrested and three firearms were discovered during the bust ... including two handguns and a rifle. It's unclear which gun cops have associated with Maroney. "