Four months after the show was taped, it is unclear if a wedding will actually take place.
The 29-year-old San Francisco winemaker offered Robertson, 28, a Neil Lane engagement ring on a mountain in Zermatt, Switzerland, as he told her, "You are my forever."
A clearly happy Robertson accepted his proposal, beating out finalist Lindzi Cox for the honor of being Flajnik's bride.
Monday night's finale was taped in November and the winner of the competition kept secret.
However, Robertson tearfully revealed during the taping of the "After the Final Rose" episode that Flajnik broke up with her last month, then tried to win her back after he was photographed kissing three other women.
Flajnik insisted the women were just friends and the pictures were old. He said he is now committed to making his relationship with Robertson work. He even gave her back the engagement ring, which she accepted.
"In the last couple of weeks we've been able to talk," he said.
"I'm going to be there for you forever," Flajnik promised Robertson. Robertson said, however, she is not sure she can ever really trust Flajnik again.