The Spokesman-Review reports an internal investigation discovered Kenoyer had sex with the same woman linked to a Spokane police officer who resigned in June.
The sheriff says Kenoyer was honest but declined to sign a "last chance agreement" that would have reinstated him under strict guidelines.
Kenoyer began working for the sheriff's office in 1998 in the jail and has been a school resource officer and patrol deputy. He's the second Spokane County sheriff's deputy to leave this week. Det. Dale Toliver resigned after he was arrested in a domestic violence case.