The photo shows five-month-old James' tiny little fingers wrapped around Reynolds' thumb, reported Us magazine.
The shot, first shared by Reynolds, 38, included the caption. "Thumb wrestling isn't always fair. Curiously, my hand is the small one."
Lively, 27, quickly regrammed her husband's social media update and added under her account. "...And with that, my husband @vancityreynolds joins Insta."
James is Reynolds and Lively's first child.