Police spokesman Officer David Pubins says the allegation involves an incident that took place on Jan. 13.
Pubins says police are interviewing those involved and processing any physical evidence to determine if criminal charges are appropriate. More details weren't available.
Team officials did not immediately return phone messages seeking comment Tuesday. It wasn't clear if Beasley had legal representation.
Beasley was Phoenix's biggest offseason acquisition in 2012 after signing a three-year, $18 million contract. He was the No. 2 overall pick of the 2008 draft.
Beasley has been in trouble before. He was cited earlier this year on suspicion of speeding, driving on a suspended license and driving without a license plate or registration.