The Seattle Times reported that prosecutors say David Blackmer, 44, vowed to "ruin" the woman's life after she went to his Snohomish County home last July to tell his wife about the affair.
He also was ordered Wednesday to pay a $1,000 fine and avoid contact with the victim for five years.
He will be allowed to carry a gun.
The Seattle Police Department's Office of Professional Accountability will now begin a separate administrative investigation into the detective's conduct.
Blackmer has been on leave.
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