"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. Happy 40th, my love," Biel, 38, wrote.
The post included a photo of Timberlake wearing a birthday crown and grinning at a tall stack of pancakes and fruit.
The gallery of images also showed the couple at a baseball game, awards show and golf course.
Biel and Timberlake married in 2012. They welcomed their second son Phineas in September. They are also the parents of 5-year-old Silas.