Timothy Brehmer killed mother-of-two Claire Parry, 41, in a pub car park on May 9.
Brehmer, a Dorset Police constable, admitted the manslaughter of the married nurse with whom he had been having an affair for more than 10 years.
He said her death, which occurred during a “kerfuffle” in his car, was an accident.
A jury took just under three hours to find the Pc not guilty of murder. He will be sentenced at Salisbury Crown Court by Mr Justice Jacobs later on Tuesday.
The court heard that while the pair were parked at the Horns Inn in West Parley, Mrs Parry had taken Brehmer’s phone and sent a message to his wife saying: “I am cheating on you.”
Brehmer, also 41, claimed she accidentally suffered the fatal injury while he was trying to push her out of the vehicle so that he could drive away.
The trial heard that in the days before her death, Mrs Parry had started to believe her marriage to Andrew Parry, a fellow Dorset Police officer, was coming to an end, as was her relationship with the defendant.
She had carried out research using an alias on Facebook into Brehmer and became convinced he had previously had at least two other affairs.
Mrs Parry was in contact with a police officer called Kate Rhodes, who told her she had an affair with Brehmer in late 2011. This made Mrs Parry see him “in a very different light”.
Brehmer told the jury that Mrs Parry was angry and had “uncontrolled jealousy” because she had found out about the previous affair, leaving him feeling “desperate” and suicidal.
“I couldn’t face the rejection from my family, I felt I didn’t have anyone I could talk to,” he said.
Brehmer said that when Mrs Parry drove into the car park she was angry and after she got into his vehicle she asked for his phone so she could look through his social media apps.
“She was taking the mickey out of me, she was angry, she was being snide, nasty,” he said.
“She was so angry, I do not know if she was jealous of my ‘perfect life’, as she called it.”
Brehmer said that at one point he stabbed his arm three times with a penknife but Mrs Parry “did not care”.
He said he demanded she get out of his car but she refused, so he first tried to pull her out before he “bundled” into the car to try to push her out, and his arm “must have slipped up in all the melee”.
Asked by his barrister, Jo Martin QC, if he had planned to kill Mrs Parry, he replied: “Absolutely not.
“I didn’t intend to hurt her in any shape or form.”
Under cross-examination by Richard Smith QC, prosecuting, Brehmer initially denied being a “well-practised liar” but then accepted the term after admitting he lied “consistently well” to his wife over the affair.
Asked if he described himself as a “devious b******”, Brehmer responded: “That’s how I consider myself now.”
Mrs Parry, from Bournemouth, was taken to Royal Bournemouth Hospital but died the following day.
A post-mortem examination concluded the cause of death was a brain injury caused by compression of the neck.
At the time of the incident Brehmer was seconded to the National Police Air Service based at Bournemouth Airport.
Brehmer, of Woodcock Lane, Hordle, Hampshire, had denied murder but admitted manslaughter.