The proud papa took to Instagram on Tuesday to share the exciting news, posting a snapshot of a baby bassinet overflowing with pink and white roses and carnations.
"Saturday night my wife Courtney and I welcomed into our world and family a very beautiful baby girl named Ruby Sixx," Nikki wrote in the caption. "A spunky little girl with a full head of hair making her the 5th Sixx joining her two sisters and two brothers."
The rocker is already the father of four other children -- sons Gunner, 28 and Decker, 24, and daughter Storm, 25, who he shares with ex-wife Brandi Brandt, and 18-year-old daughter Frankie Jean, from his marriage to Donna D'Errico.
"Fatherhood has been the greatest joy in my life.....and @how2girl is a real natural as a mom," he wrote, praising his lovely wife. "I would like to thank our family and friends for all the love and support ( and flowers ). We are already sleep training her with a metronome and I got her a pink bass."