According to Us Weekly, the on-off couple made the decision to marry last month.
An insider claimed that Timberlake proposed to Biel in the mountains of Jackson, Wyoming.
They added: "Justin knows how much she loves snowboarding and the mountains, so it was the perfect place."
The couple had previously split in March after a four-year relationship but were reported to have reconciled a few months later.
"When they reunited, they had a conversation about taking the next step," the alleged source added.
The pair were previously reported to have got engaged last month, but neither party confirmed or denied the rumours.
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