Kobe Bryant made the announcement on his Instagram page, where he posted a photo showing a baby outfit with "Baby Mamba" written across the chest.
He wrote, "Beyond blessed and excited to share that we are expecting our third baby girl!!! #Blessed #BabyMamba #Thankful."
Kobe Bryant ended his 20-year career with the Lakers in front of a sold out crowd at the Staples Center in April.
He scored 60 points as Los Angeles defeated the Utah Jazz 101-96.
Following the game, Kobe Bryant addressed the crowd, lastly saying, "Mamba out," before setting the mic on the ground and walking off the court.
Kobe Bryant and Vanessa Bryant already have two girls together, Gianna and Natalia.