Presenter Steve Harvey said he felt “terrible” for the blunder which was broadcast live on TV.
He mistakenly announced Miss Colombia had won the pageant with the Philippines contestant as runner-up.
But Harvey then had to admit to the audience he had made a gaffe.
It meant Ariadna Gutierrez Arevalo, 21, of Colombia, who had already been crowned, had to take off the glitzy trinket and hand it to Pia Alonzo Wurtzbach, a 26-year-old actress and model from the Philippines.
Harvey wrote on Twitter: “I’d like to apologise wholeheartedly to Miss Colombia and Miss Philippines for my huge mistake.
“I feel terrible.”
Wurtzbach appeared to take the mix-up in her stride.
Speaking afterwards, she said: “It’s a very non-traditional crowning moment. It is very 2015.”
It was the first Miss Universe title won by a contestant from the Philippines for more than 40 years.
It was also the first time viewers had a chance to vote on the winner, rating contestants in the swimwear, evening gown and interview competitions.
Shortly before the end of the contest, a car mounted a pavement close to the Las Vegas venue and ploughed into pedestrians, killing one person and injuring at least 37 others.
Police have said the fatal crash was intentional.