A crafty dog owner turned detective to catch a young man savagely attacking her boxer — she used a secret camera to record the beatings, authorities said Tuesday.

The Washington Heights woman sublet a room to Bryan Li in July and soon began to notice her dog quivering whenever Li entered.

She set up the video camera room that captured Li “repeatedly striking and hitting the dog,” a Criminal Court complaint states.

Li has been charged with aggravated cruelty to animals.