The incident happened just before 9:30 p.m. at the store on Sunset and Vine.
The off-duty officer was working as private security at the store when a known trespasser went in and was asked to leave, police said. That's when the suspect kicked the officer in the head.
The officer was taken to the hospital, and the suspect was arrested about a block away.
The crime happened at the same Walgreens where a security guard shot a man last year.