The former backup dancer, 36, and his wife, Victoria Prince, are also parents toJordan Kay, 2½.
The proud papa announced the arrival of his baby girl by posting a photo of an "It's a Girl!" wagon filled with flowers.
"Words cannot express the joy my children bring me. We are very excited to welcome the newest member of our family," he writes. "I love you with all my heart [Victoria]. #thefedz #sixthman I mean #sixthwoman."
Federline is also dad to sons Jayden Jaymes, 7½, and Sean Preston, 8½, with ex-wife Britney Spears, and Kaleb, 9½, and daughter Kori, 13, with ex-girlfriend Shar Jackson.