Justin Timberlake is celebrating his wife, Jessica Biel, on her birthday.

The singer/actor, 40, penned a sweet message to Biel as she turned 39, calling her his “favourite person in the world.”

“Happy Birthday to my favourite person in the world,” Timberlake’s post began. “The strongest, funniest, best, brightest and most beautiful mom/partner/BOSS there is. I’m lucky to learn from you and laugh with you and celebrate you every day. But, today we do it with [cake]!”

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He continued, “Here’s to more slow-mos in the wind, jumping into bushes to save our kid’s toys, and timing out jump pics perfectly… I love you with all that I know.”

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The couple, who wed in 2012, share two sons together, Silas, 5, and Phineas, 1, who they secretly welcomed last year.

“His name is Phineas, and he’s awesome and so cute, and nobody’s sleeping,” Timberlake confirmed to Ellen DeGeneres earlier this year. “But we’re thrilled. We’re thrilled and couldn’t be happier. Very grateful.”