He also posted a photo of himself with the newborn on his social media accounts.
"Blessed to have a healthy baby boy! Marcus Thomas Morris Jr. Future Hall OF Fame. 7 pounds 8 ounces 21 inches," Morris wrote on Instagram, including several photos of his family's newest addition.
On July 13, Morris cryptically tweeted out the due date, writing: "July 20, 2018," with no other context.
Morris' twin brother Markieff Morris retweeted his brother's "he's here" message. Markieff welcomed his daughter Jyzelle in September of 2017.
Marcus Thomas Morris Jr. 7/20/18 pic.twitter.com/vznux4UxmM— Marcus Morris (@MookMorris2) July 20, 2018
The Celtics family keeps growing. Forward Al Horford and wife Amelia Vega welcomed their third child on July 11. Center Aron Baynes and his wife welcomes their second child five days later. Celtics star forward Gordon Hayward and his wife Robyn announced in July that they are expecting their third daughter in January.
He’s here!— Marcus Morris (@MookMorris2) July 20, 2018
Marcus, 28, averaged 13.6 points and 5.4 rebounds per game last season in 54 appearances for the Celtics. He will make $5.3 million this season in the final year of his contract with the Eastern Conference power.