The Colorado football head coach opened up on the new addition to his fam at a press conference on Friday ... saying his daughter, Deiondra, just gave birth -- the same day he turned 57.

"That’s how good God is," Sanders said. "God would choose me on my birthday, for my daughter, of my kids, to have their first child, the first time I’m a grandfather, on my birthday."

"Do you understand how beautiful that is? How much of a blessing that is?"

Sanders -- who has a total of five kids -- also had a message for Deiondra during his presser.

"God bless you, Deiondra. I love you, baby girl."

Deiondra -- who hasn't addressed the news just yet -- announced she was having a baby with R&B star Jacquees in March. They got engaged at their baby shower last month.

Outside of his grandson being the best bday gift ever, Sanders was also serenaded by his team at practice ... and Coach Prime told them what he REALLY wants from them.

"My wish is that you all become professionals," the Hall of Famer said. "Not just professional athletes but professionals. You become wonderful fathers, wonderful men, wonderful women as well."

Sanders -- who spent 14 seasons in the pros -- will be entering his second season as Colorado football's HC this fall.