The pair dated on and off for more than four years before Timberlake proposed in the mountains of Jackson, Wyo., the report said.
"Justin knows how much she loves snowboarding and the mountains, so it was the perfect place," one insider told the magazine about the location where Timberlake asked for Biel's hand in marriage. "When they reunited, they had a conversation about taking the next step."
A source close to Timberlake told UsMagazine.com he's "never been happier" with Biel.
"He knew it was the right time to propose," the source said.