The couple have been rumoured to be engaged for some time, with Timberlake's grandmother apparently confirming the news in January.
Biel described Timberlake as her "fiancé" on The Tonight Show with Jay Leno last week and has now admitted that she never expected to get engaged.
"It's actually not anything I ever really thought was going to happen to me or that I really wanted," The AP quotes her as saying. "And when it happened it just seemed - it just was right."
Biel also recently revealed that Timberlake picked out her engagement ring and praised him for his "real eye for design".
Elsewhere, Timberlake's former *NSYNC bandmate Lance Bass has confessed that he will "probably cry" at his friend's wedding.