The 27-year-old actress marked the occasion by sharing a photo of the 38-year-old actor with six-month-old daughter James on Instagram.
"Happy Fathers Day!!!" she wrote before joking, "@vancityreynolds Since the day our baby was born, I've felt so strongly in my heart that you were most likely the father. #ILoveYouSoMuchItsSilly."
Reynolds also poked fun on Twitter, writing, "On Fathers Day, my daughter smiled at me. It was all the gift I needed as I packed a bag and left for 6 years to write a book on parenting."
Lively and Reynolds met on the set of Green Lantern in 2010, and married in Sept. 2012. The couple welcomed James in Dec. 2014, and the actress later said she is able to juggle motherhood and her career because she doesn't force herself to choose between the two.
Lively is slated for new Marc Forster (World War Z) thriller All I See is You and an upcoming Woody Allen movie. Reynolds is scheduled for Tarsem Singh (The Fall) sci-fi film Self/less with Ben Kingsley, and Marvel solo movie Deadpool.