Andrew Smith who has been with the Janesville police department for 5 years, was arrested for three criminal charges.
Police say Smith got into a argument with his mistress Cassandra Tousignant who was also an officer in her probationary period.
While visiting Tousignant, Smith took out a knife and struck her clock radio.
Later that day Smith drove to Tousignant's house, police say he was intoxicated and showed a handgun.
The altercation continued later that night when Smith threw Tousignant on the floor and she suffered minor injuries. After an investigation the police department asked Smith to resign.
"It's clearly unacceptable. Our police officers know that, they know that there are very significant consequences you can lose your job over it, you can be criminally prosecuted if you're involved in criminal activity and that's what occurred here," Police Chief Neil Mahan said.
Police say Tousignant's resigntation had to do with her failing to meet standards during her probationary period.
Smith was charged with two counts of disorderly conduct while armed and one for disorderly conduct.