The 40-year-old singer and actor marked the occasion Wednesday by sharing a loving tribute to Biel on Instagram.
Timberlake posted a slideshow of photos, which included a picture of himself kissing Biel on the cheek. Timberlake and Biel married in October 2012 and have two sons, Silas and Phineas.
"Happy Birthday to my favorite person in the world... the strongest, funniest, best, brightest and most beautiful mom/partner/BOSS there is," Timberlake captioned the post. "I'm lucky to learn from you and laugh with you and celebrate you every day. But, today we do it with [cake]!!!"
"Here's to more slow-mos in the wind, jumping into bushes to save our kid's toys, and timing out jump pics perfectly..." he said. "I [love] U with all that I know."
Biel voiced her love for Timberlake while celebrating the singer's birthday in January.
"There's no one I have more fun with, have more laughs with, feel more deeply for, and have more history with. I honor you today, baby. And wish you the most creative and fulfilling year to date," she wrote.
"He's awesome and he's so cute," Timberlake said. "Nobody is sleeping, but we are thrilled."
In February, Timberlake apologized to his ex-girlfriend Britney Spears and his fellow Super Bowl XXXVIII performer Janet Jackson for his past behavior. Timberlake said he's "benefited from a system that condones misogyny and racism."
"I have not been perfect in navigating all of this throughout my career," the singer said. "I want to take accountability for my own missteps in all of this as well as be a part of a world that uplifts and supports."
"I care deeply about the wellbeing of the people I love and have loved. I can do better and I will do better," he vowed.
Justin Timberlake turns 40: a look back