Villanueva served as L.A. County sheriff from 2018 until 2022, when he lost to Robert Luna, who already declared he's running for reelection.
Villanueva often clashed with county supervisors when he was sheriff.
In a statement declaring his candidacy, he said he wants to take politics out of public safety.
"This campaign is about building a safer, stronger future," the statement said. "I've walked this path before, and I'm returning with broader experience, more clarity, and a deeper commitment to the people who deserve better."
Oscar Martinez, a sheriff's lieutenant at the Palmdale station, also announced he is running for sheriff.