"Being a dad is better than I ever could have imagined," the 40-year-old captioned a series of snaps, which included a photo of the "Cry Me A River" singer playing video games with the couple's eldest son Silas, 6, while Phineas held a controller.
"Thankful to my dads and my grandfathers for leading the way, making the sacrifices for me to live out my dreams, and for teaching me that REAL LIFE happens in all the little moments. Happy Father’s Day to all the dads out there!!!" he added.
Fans and friends took to the comments to wish the father-of-two a happy Father's Day.
"Happy Father’s Day ! Bless up !" DJ Khaled commented.
"OMG PHINEAS REVEAL ☺️," one fan gushed.
"happy Father’s Day Justin !! hope you enjoy your day along with the others <3," another wrote.
His wife also posted a sweet tribute.
"Baby, you are the moon, the stars and the sour cream pound cake of my life," Biel, 39, captioned a series of snaps of her husband.
"You put a roof over our heads, and then fix it when the home made rocket goes terribly wrong. You put food on the table, so the baby can throw it under the table. You let us use the couch as an indoor trampoline even though we have one outside. You love the noise and the chaos of it all even though you’ve worked all night are are trying to sleep," the 7th Heaven alum continued.
The post concluded, "We are grateful for you for all the big things and the things we don’t see. Thank you for making it so fun. We love you dearly. And to all the other dads out there taking names and building play sets, I salute you. Happy Father’s Day."
The news that the pair had welcomed their second child broke in July when Brian McKnight spilled that his friend had a "new baby." Months later, *NSYNC bandmate Lance Bass confirmed the exciting news — but noted Timberlake "would kill" him if he revealed the baby's name or gender.
As OK! previously reported, Biel revealed that she and Timberlake didn't intend to keep her pregnancy and Phineas' arrival a secret. "It was just, COVID happened, and then I went to Montana with my family and never left," she explained during her June 14 appearance on Dax Shepard’s "Armchair Expert" podcast.