What Happens in Vegas is a 2008 romantic comedy film from 20th Century Fox starring Cameron Diaz and Ashton Kutcher.

In the city of New York, highly-strung stockbroker Joy (Cameron Diaz) is dumped by her fiance, while laid-back carpenter Jack (Ashton Kutcher) is fired from his job. Emotionally distraught, they both travel to Las Vegas where they meet in a hotel room that had been double booked. After clearing up the misunderstanding at the reception, they proceed to engage in a night of partying resulting in the two getting married in an inebriated ceremony. Soon they decide to divorce, at which time Jack takes a quarter from Joy and drops it into a slot machine as she walks away. He hits a $3,000,000 triple-jackpot. They were supposed to go on their separate ways but due to the money the two must figure out a way to share it since they are still legally married. Once back in New York, a judge declares that the couple cannot divorce until they attempt to co-exist for six months, while being monitored by a marriage counselor(Queen Latifah) weekly. They will be permitted to keep half the winnings from Las Vegas if they cooperate. If either party does not, their fortune will be confiscated. As the newlyweds devise more and more cunning schemes to undermine one another and keep the other's share, Jack and Joy ironically find themselves developing an unexpected attraction to one another...

Twentieth Century Fox held the world premiere of What Happens in Vegas on May 1, 2008 at the Mann Village Theatre in Westwood, CA. Its London premiere was at the Odeon in Leicester Square, London. It opened in wide release in the United States on May 9, 2008.

Big Brother 9 housemates Ryan and Sharon won a special screening of the film after winning a "luxury competition" in April. They were also awarded tickets to the May 1 Hollywood premiere.

According to Video eta What happens in Vegas will be released on DVD and Blu-ray August 26, 2008. The DVD will be a 2-disc special edition with a digital copy only.

The film received generally negative reviews from critics. As of May 11, 2008, the review aggregator Rotten Tomatoes reported that 27% of critics gave the film positive reviews, based on 95 reviews — with the consensus that the film "mostly settles for derivative romantic comedy conventions." Metacritic reported the film had an average score of 36 out of 100, based on 31 reviews.

In its opening weekend, the film grossed $20,172,474 in 3,215 theaters in the United States and Canada, ranking #2 at the box office, behind Iron Man, but ahead of the highly-anticipated Speed Racer and averaging $6,274 per venue. As of June 21, 2008, it has grossed a total of $200,395,149 worldwide — $78,344,563 in the United States and Canada and $122,050,586 in other territories, which already makes the film a huge financial success, considering its modest budget of only $35 million.

An outtake was leaked on YouTube and shows a scene where the characters portrayed by Diaz and Kutcher are getting married while intoxicated. The two were actually intoxicated in order to make the scene look realistic, but their erratic behavior caused the scene to be filmed multiple times.