According to a statement issued by a spokesperson for the department, Villanueva is experiencing mild symptoms.
"Regrettably, the Sheriff has tested positive for Covid. He has mild symptoms but is in good spirits," read the statement.
It's unclear how Villanueva may have contracted the virus.
The 59-year-old is vaccinated against COVID, though it's unclear if he has received a booster shot.
Villanueva is currently running for reelection.
After a tight race in the June 7 primary election, he's headed to a November runoff against retired Long Beach Police Chief Robert Luna.
The sheriff has long battled the Los Angeles County Board of Supervisors on multiple issues, including a vaccine mandate in his department.