The 53-year-old actress shared a picture with baby Billy Wednesday on Instagram after Stamos welcomed the infant with wife Caitlin McHugh.
The picture shows Loughlin smiling down at Billy as she cradles the newborn in her arms. The snapshot appears to be from her first meeting with the infant this month.
"Move over @johnstamos there's a new kid in town! Billy The Kid and he's already stealing [hearts]! #iminlove," Loughlin captioned the post.
"The Tin Man, Dorothy and The Scarecrow say hi to the cutest of Munchkins #BillyandtheRippers," the 54-year-old actor wrote at the time.
McHugh gave birth to Billy in April. Stamos dedicated a sweet post to the 32-year-old actress this week on her first Mother's Day as a mom.
"#HappyFirstMothersDay/Birthday!! As if it were even possible, when you became a mother, you became even more beautiful," the star gushed. "You've brought more love and joy to my life and I ever imagined. Thank you from the luckiest guy on the planet. XOJS @caitlinskybound."
Stamos, Loughlin and Saget play Uncle Jesse, Aunt Becky and Danny Tanner on Fuller House. The Netflix series is a sequel to the ABC sitcom Full House, which had an eight-season run from 1987 to 1995.