Timberlake made the announcement on his birthday with a sweet Instagram photo of himself kissing Biel's belly.
"Thank you EVERYONE for the Bday wishes! This year, I'm getting the GREATEST GIFT EVER. CAN'T WAIT," he wrote on Saturday's pic.
He even revealed the couple do not know they baby's gender yet with a series of hashtags. "#BoyOrGirl #YouNeverKnow #WeDontEvenKnow #WeAreTakingBets."
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Biel has been trying to hide her baby bump for months behind baggy clothes but the secret was out when friends began congratulating her on social media and Timberlake's former bandmate Joey Fatone confirmed the news.
"He told me a while ago, and I kept my mouth shut," Fatone revealed in December. "But now I can talk about it!"
The couple married in October of 2012 and this will be their first child.