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. He is the only member to remain throughout their entire history.

Prior to forming Mötley Crüe, Sixx was a member of Sister and then went on to form London with his Sister bandmate Lizzie Grey. In 2000, he formed the side project group 58 with Dave Darling, Steve Gibb, and Bucket Baker, releasing one album, Diet for a New America. In 2002, he formed the hard rock supergroup Brides of Destruction with L.A. Guns guitarist Tracii Guns. Formed in 2006, his side band Sixx:A.M. featured songwriter, producer, and vocalist James Michael and guitarist DJ Ashba, initially to record an audio accompaniment to Sixx's autobiography The Heroin Diaries: A Year in the Life of a Shattered Rock Star.

Sixx has worked with numerous artists and groups, co-writing and/or producing songs with artists such as Sex Pistols guitarist Steve Jones, Lita Ford, Alice Cooper, Meat Loaf, Marion Raven, Drowning Pool, Saliva, and The Last Vegas.

In 2006, Sixx launched the clothing line "Royal Underground" with Kelly Gray, initially focusing on men's clothing before expanding into women's apparel.

In 2010, Premiere Radio Networks launched nationally syndicated rock/alternative music radio programs "Sixx Sense" and "The Side Show Countdown", both hosted by Sixx and co-hosted by Jenn Marino.