The mother of two also updated her Instagram bio to read, "Mama of Tatum & River," while Johnson posted the same hospital photo to his Instagram account.
"I'm so thankful for my family and the new addition River Jones Johnson," he wrote.
After the birth, Johnson flew from California to Wisconsin compete in this year's U.S. Open. As the pro golfer prepares to defend his championship title, he reflected on the last year by sharing another Instagram photo holding his U.S Open trophy.
"What a great year it's been with the trophy!" the 32-year-old athlete gushed. "So many great things have happened especially Monday afternoon with our second son being born. @paulinagretzky I love you."
In February, Paulina, who is the daughter of hockey great Wayne Gretzky, revealed that she and Johnson were expecting their second child together with a precious pregnancy photo captioned, "Coming soon."