Ashley Hamilton, an actor who was once married to Shannen Doherty, got arrested after police say he threatened a hotel employee with a knife.

Law enforcement tells TMZ … Ashley was arrested a little after 5 AM on Aug. 24 following an incident in a posh Beverly Hills hotel, when a staffer told cops the actor pulled a knife and took a fighting stance.

Cops say Ashley was asleep on a couch in the hotel lobby when the employee woke him up and told him he couldn’t sleep there … and we’re told Ashley started arguing, and things quickly escalated.

Police say Ashley brandished a pocketknife, and the employee took off running to call 911. When cops arrived, we’re told they found Ashley and then recovered the knife in the lobby.

Law enforcement says the hotel staffer wanted to press charges — so Ashley was arrested for misdemeanor brandishing and was taken to a local hospital to get checked out for possibly being under the influence.

Ultimately, cops say Ashley was given the all-clear at the hospital, and officers then cited and released him.

Ashley tells TMZ … the employee startled him, and that’s why he pulled the knife, though he barely even remembers the incident.

In the hospital, Ashley says he told doctors he took some pills, including oxycodone, which he says he was prescribed as a result of a cancer diagnosis.

Ashley, son of actor George Hamilton, says he’s battling squamous cell skin cancer and has undergone two surgeries and treatments … and that’s why he’s taking pain pills.