After announcing Miss USA, Olivia Jordan, as the third runner-up, Harvey initially named Miss Colombia, Ariadna Gutierrez Arevalo, as the night's big winner, relegating the remaining contestant, Miss Philippines, Pia Alonzo Wurzbach, to the second runner-up spot.
But after Arevalo received the crown and a bouquet of flowers, Harvey announced he had made an error and that Wurzbach was in fact the winner.
A stunned Wurzbach, who had taken her place with the other contestants behind Arevalo, then walked out to center stage. Last year's Miss Universe, Colombia's Paulina Vega, then removed the crown from Avrevalo's head and placed it on Wurtzbach.
With both pageant contestants standing at the front of the stage, the show concluded.
Harvey, who was hosting the contest for the first time, held up the card announcing the order of finishers for Fox network cameras to see up close afterward. He said he re-read the card and noticed it said "first runner-up" next to the Colombia contestant's name before he asked producers if he had made a mistake. Talking with reporters afterward, Harvey and an executive for pageant owner WME-IMG called it human error.
"Nobody feels worse about this than me," Harvey said.
In addition to his on-air apology, Harvey expressed contrition on social media. However, he initially misspelled the home countries of both contestants before also fixing that.
Wurtzbach later said she felt conflicting emotions as the mistake happened: joy when she was told she had indeed won, concern for Aravelo and confusion at the whole situation.
Wurtzbach said she tried to approach Aravelo onstage afterward but the Colombian was crying and surrounded by a crowd of women. She said she realized it was, "probably bad timing."
"I did not take the crown from her," Wurtzbach told reporters after the pageant concluded, saying she wished the contestant from Colombia well and hoped the Latin American community understands that "none of this was my fault."
"None of this was done on purpose. It was an honest mistake," she said, apologizing on behalf of the organization she now represents. She said Harvey told her afterward that she "should just enjoy the moment."
Wurtzbach was later shown laughing in a video posted on the Miss Universe Twitter.
“It’s a very non-traditional crowning moment, isn’t it?" she said. "It’s very, very 2015."
Meanwhile, Aravelo was seen wiping away tears in a video posted by the organization.
“Everything happens for a reason, so I’m happy for all [that] I did," Aravelo said. "Thank you for voting for me. Thank you…”
The Miss Universe organization issued a statement late Sunday to apologize for the mix up.
"The excitement of live TV was evident tonight on The Miss Universe stage with over 10 million live fan votes tabulated. Unfortunately, a live telecast means that human error can come into play. We witnessed that tonight when the wrong winner was initially announced. Our sincerest apologies to Miss Universe Colombia 2015, Ariadna Gutierrez-Arévalo, Miss Universe Philippines 2015, Pia Alonzo Wurtzbach, their families and fans."
Even before Sunday night's oops moment, the pageant was involved in another controversy when a backlash against the pageant's former owner Donald Trump led Univision to pull out of the broadcast and the businessman to sell it in September.
Trump was quick to weigh in on Sunday's error, declaring the mistake would "never have happened" during his time as owner.
NBCUniversal and Trump co-owned the Miss Universe Organization until earlier this year. The real-estate developer offended Hispanics in June when he made anti-immigrant remarks in announcing his Republican presidential run.
The competition started with women representing 80 countries between the ages of 19 and 27. For the first time, viewers at home weighed in, with their votes being tallied in addition to four in-person celebrity judges.
Shortly after Sunday night's confusion, Colombian President Juan Manuel Santos tweeted a message on his official account to Ariadna Gutierrez. "For us, you will continue being miss universe! We are very proud!"
Philippines presidential spokesman Edwin Lacierda didn't address the controversial win but said, "in bagging this victory, Ms. Wurtzbach not only serves as a tremendous source of pride for our people, but also holds up the banner of our women and of our country-as a true representative of what the Filipina can achieve."
It's the third time a contestant from the Philippines has won the title. It could have been the second win in a row for Colombia.
As all this was unfolding, a car drove up onto a sidewalk and struck dozens of people just outside the Planet Hollywood hotel-casino where the pageant was taking place. The Las Vegas Strip was soon jammed with ambulances and fire trucks, and authorities said 37 people were taken to a hospital to be treated for injuries and one person was killed.
The Associated Press contributed to this report.