"I've always been a believer in second chances," Trump, who owns the Miss Universe Organization with NBC, said with Conner at his side.
Trump said he and Conner had met earlier Tuesday morning.
"She left a small town in Kentucky and she was telling me that she got caught up in the whirlwind of New York," Trump said at a news conference. "It's a story that has happened many times before to many women and many men who came to the Big Apple. They wanted their slice of the Big Apple and they found out it wasn't so easy."
Conner won the title in April and has been living in New York. Recent media accounts of heavy drinking brought a storm of criticism because she was underage at the time. She turned 21 on Monday.
In a tear-choked voice, Conner said, "In no way did I think it would be possible for a second chance to be given to me."
Turning to Trump, she said, "You'll never know what this means to me, and I swear I will not let you down."
Trump said Conner would be entering rehab. A pageant official said details would be worked out privately with Conner over the next weeks.
"I think Tara is going to be the great comeback kid," Trump said.
If Conner had been dethroned, her title would have been taken over by first runner-up Miss California Tamiko Nash.
Conner, a 5-foot-5 blond, has been competing in pageants since age 4. After winning the Miss USA title in April, she finished fourth in the Miss Universe pageant in July.
In 2002, Miss Russia Oxana Fedorova won the Miss Universe pageant but was stripped of her title after violating her contract. Trump said Fedorova didn't show up for some photo shoots and charity events. It was the first time a titleholder had been ousted in the contest's more than 50-year history. Fedorova denied she was fired and said she gave up the title voluntarily.