Justin Timberlake might have an ace in the hole to get his DWI case dismissed ... and it's a woman who took the keys to his car after his arrest ... TMZ has learned.

As we reported, Justin was busted for drunk driving last month, after partying with friends at a Sag Harbor hotel. Cops say they saw him blow a stop sign and swerve as he was following some friends.

After he was stopped, cops say he failed multiple sobriety field tests and also smelled of alcohol. He refused to blow into a Breathalyzer, and claimed he only drank one martini.

Sources with direct knowledge tell TMZ ... the car Justin was following came back to where the singer was pulled over. There was a woman and her husband in that car, and the woman got out and went up to one of the cops and began questioning him, asking, "Are you going to arrest Justin Timberlake?"

She then offered to drive Justin to their house, where he was going to spend the night ... in other words, just let him go and she and her husband would get him home safely. That strategy did not work, and Justin was arrested.

Now here's the twist. Our sources say the woman asked cops if she could take Justin's rental car to her house, given that Justin was going to jail. The cops obliged, she got in Justin's car and left.

We're told the woman had arrived at The American Hotel 2 hours before Justin and was drinking ... and it was apparent she had been drinking. Yet the 2 cops who were on the scene let her take the wheel and drive away.

We're told Justin's lawyer, Edward Burke, Jr. -- who is trying to get the case dismissed -- will argue the 2 young cops were so off in their judgment, they let a person who had clearly been drinking drive away, right in front of their eyes. If they were unable to properly evaluate her, how could they properly evaluate him?