Sebastian Philip Bierk (born April 3, 1968), known professionally as Sebastian Bach, is a Canadian-American singer who achieved mainstream success as the frontman of the hard rock band Skid Row from 1987 to 1996. He has acted on Broadway and has made appearances in film and television such as Trailer Park Boys, The Masked Singer, and Gilmore Girls. He continues his music career as a solo artist.

Skid Row initially formed in the mid-1980s with lead vocalist Matt Fallon fronting the group, and they began playing at various clubs around the New Jersey area. Fallon left the band in 1987, leaving Skid Row in search of a lead vocalist. Bach was spotted singing at rock photographer Mark Weiss's wedding by Jon Bon Jovi's parents, who subsequently approached him and suggested he get in touch with their son's friend, Dave Sabo, who was looking for a lead vocalist for his band. Bach was wary of joining another US-based band after briefly being a member of Detroit's Madam X. Upon hearing Skid Row's demo tapes, however, Bach flew to New Jersey to audition and became the up-and-coming band's lead vocalist in early 1987.

Skid Row achieved commercial success in the late 1980s and early 1990s, with its first two albums Skid Row (1989) and Slave to the Grind (1991) certified multi-platinum, the latter of which reached number one on the Billboard 200. The band's third album Subhuman Race (1995) was also critically acclaimed.

Following disagreements over musical material and infuriating Bolan, Bach was fired from the band in 1996. However, rumors circulated that he had left the band due to his other band-mates believing they should not play as an opening act for Kiss. Four years later, Skid Row was one of the opening acts for the 2000 Kiss Farewell Tour, without Bach.

In 1993, he shortly toured with Triumph after the release of their album Edge of Excess, providing lead vocals up until the band broke up later the same year. In 1996, he joined The Last Hard Men, an alternative rock supergroup formed by Breeders lead guitarist Kelley Deal, which also included Frogs guitarist Jimmy Flemion and Smashing Pumpkins drummer Jimmy Chamberlin. The group recorded a full-length self-titled album and recorded a cover of the Alice Cooper song "School's Out" for the soundtrack album of the 1996 film Scream.

In 1999, he released his debut solo album Bring 'Em Bach Alive!, his first release after his departure from Skid Row. In 2000, he began performing in Broadway productions. He made his Broadway debut with the title role(s) in Jekyll & Hyde in April 2000. He also appeared as Riff Raff in The Rocky Horror Show in 2001.

In early 2002, he became the host of VH1's Forever Wild. From 2003 to 2007, he had a recurring role on the WB television show Gilmore Girls as "Gil", the lead guitarist in Lane Kim's band, Hep Alien. He starred in the 2006 VH1 reality show Supergroup, in which he, along with Ted Nugent, Scott Ian, Jason Bonham and Evan Seinfeld, were tasked with forming a heavy metal supergroup, with Bach as the lead vocalist. He competed on the 2007 MTV reality show Celebrity Rap Superstars, the CMT reality show Gone Country, and the 10th season of The Masked Singer in fall 2023.