The mum-of-three and her cage-fighter love – crowned winner of Celebrity Big Brother last week – tied the knot in a "private and simple" ceremony at 16.00 local time on Tuesday in the Wynn Hotel's marriage chapel.
It came just hours after the pair landed in Sin City, where they checked into the Planet Hollywood Hotel on Sunset Strip.
After stepping out to buy wedding rings (pictured) and enjoying champagne and strawberries together, they headed off for the service, emerging 30 minutes later holding their marriage licence.
"She (Katie) had the idea while Alex was in the Celebrity Big Brother house and decided to just run with it," a source told the Mirror.
In contrast the Katie's first big fairytale wedding to ex-husband Peter Andre in September 2005, the marriage was a simple affair, only slightly delayed by the hunt to find the bride something suitable to wear.
"They wanted to have the ceremony as soon as they got to Vegas, but the bridal stores weren't open so she couldn't get a dress," the source added. "She had her fashion team trawling all over own for a gown.
"It was nothing like last time when she'd spent months designing her perfect dress… Katie wanted a no-nonsense do this time, with just her nearest and dearest."
Katie, 31, and 34-year-old Alex – due back in the UK later this week – have been dating for around seven months.
Their relationship hit a bump two months ago when the glamour model ended their romance live on air while taking part in I'm A Celebrity…, but the pair reunited and have since been talking about starting a family together.
In a statement, the newly-weds confirmed their happy news, saying: "We are very much in love and look forward to the future together."
"We can't wait to get back and celebrate our marriage with our friends and family who we know fully support our wishes."
Their publicist added: "Their decision to marry has not been made with any pre-conceived commercial plan or media deal in place, and their reason for getting married is purely down to their love for each other."