1:19 PM PT -- Withey's attorney Alan Jackson tells TMZ Sports, "This was an apparent pathetic attempt at a shakedown that we saw a mile away and we are thankful the DA recognized that."

Utah Jazz center Jeff Withey will NOT be prosecuted for domestic violence -- despite the fact cops believe there WAS a physical altercation with his Playboy Playmate ex-fiancee.

TMZ Sports has learned ... Withey was investigated by the Manhattan Beach PD after Kennedy Summers filed a police report accusing him of beating her up.

Investigators found evidence that Withey and Summers got into a physical altercation during an argument in July 2016 ... with Withey admitting he used force against Summers, which left bruises on her thigh.

But -- and this is the key part of the investigation -- cops believe the evidence shows Summers initiated the altercation by jumping on him and hitting him after accusing Withey of cheating on her.

Officials say they believe Withey has a "reasonable claim of self-defense in that he was trying to get [Summers] off him." They also say Withey "never admits to striking [Summers] out of anger."

Summers did have bruises on her thigh from the incident but no other visible injuries.

We're told the findings were brought to the L.A. County District Attorney's Office -- and the decision was made to NOT move forward with the case. In short, Withey is off the hook.