Nikki Sixx (born Frank Carlton Serafino Feranna Jr.; December 11, 1958) is an American musician, best known as the co-founder, bassist, and primary songwriter of the heavy metal band Mötley Crüe, being the only member to remain throughout their entire history.Prior to forming Mötley Crüe, he was a member of Sister before going on to form London with his Sister bandmate Lizzie Grey.

In 2000, he formed side project group 58 with Dave Darling, Steve Gibb and Bucket Baker, issuing one album, Diet for a New America. Also in 2002, he formed the hard rock supergroup Brides of Destruction with L.A. Guns guitarist Tracii Guns.

His side band Sixx:A.M. was formed in 2006, initially to record an audio accompaniment to Sixx's autobiography The Heroin Diaries: A Year in the Life of a Shattered Rock Star, which featured songwriter, producer, and vocalist James Michael and guitarist DJ Ashba.

He launched the clothing line "Royal Underground" in 2006 with Kelly Gray, formerly the co-president and house model of St. John. In 2010, Premiere Radio Networks launched nationally syndicated rock/alternative music radio programs "Sixx Sense" and "The Side Show Countdown" with both based in Dallas, Texas and hosted by Sixx and co-hosted by Jenn Marino.

He was married to his first wife, Playboy Playmate Brandi Brandt from May 1989 to November 1996; they have three children: Gunner Nicholas Sixx (born January 25, 1991), Storm Brieann Sixx (born April 14, 1994), and Decker Nilsson Sixx (born May 23, 1995). One month after the divorce from Brandt, he married his second wife, actress and Playmate of the Month for September 1995, Donna D'Errico; they have one daughter, Frankie-Jean Mary Sixx (born January 2, 2001). They separated twice and divorced in June 2007.

He dated tattoo artist Kat Von D from 2008 to 2010. He then married his third wife, Courtney Binghamton on March 15, 2014; they have a daughter Ruby Sixx (born July 27, 2019).