Guy Theodore Sebastian (born 26 October 1981) is an Australian singer, songwriter and record producer who rose to fame after winning the first season of Australian Idol, in 2003. Born in Malaysia and raised in Adelaide, Australia, Sebastian has since released eleven top ten albums, including three number ones. Eight have gained either platinum or multi-platinum certification. He has also achieved twenty three top twenty singles, with fourteen of them, reaching the top ten, including six number ones. He is the only Australian male artist in Australian chart history to achieve six number one singles, and places third overall for all Australian acts. Twelve of his singles have been certified multi-platinum, including the 14× platinum "Battle Scars". His debut single "Angels Brought Me Here" was the highest selling song in Australia in the decade 2000 to 2009. With eighty-two platinum and seven gold certifications and combined album and single sales of over six million in Australia, he has the highest certifications and sales of any Australian Idol contestant.

Sebastian has worked with a number of notable American musicians, including Brian McKnight, Robin Thicke, Steve Cropper, John Mayer, Jordin Sparks, Eve and Lupe Fiasco. "Battle Scars", a collaboration with Fiasco, spent 20 weeks on the US Billboard Hot 100 chart, peaking at number 71 and achieved 3x platinum certification. "Battle Scars" also reached number two in Norway and New Zealand. Sebastian has reached the top ten in New Zealand with an album and six singles, including two number ones, and gained eleven platinum and five gold certifications there. In 2015, he represented Australia in its debut in the Eurovision Song Contest with the song "Tonight Again", finishing in fifth place.

Throughout his career, Sebastian has received 34 ARIA Award nominations, winning seven including Best Pop Release and Best Live Act. Other awards include the APRA Urban Work of the Year, the [V] Oz Artist of the Year, Urban Music Awards for Best Male Artist and Best R&B Album, and an International Songwriting Competition award for "Battle Scars". Sebastian has sung at many notable events, including performing for Pope Benedict XVI, Oprah Winfrey and Queen Elizabeth II. Sebastian was previously an ambassador for World Vision Australia and the Australian Red Cross. Together with his spouse Jules, Sebastian created "The Sebastian Foundation." Sebastian was made a Member of the Order of Australia (AM) in the 2019 Queen's Birthday Honours in recognition of his significant service to music and charity. He was a judge on Australia's The X Factor from 2010 to 2012 and again from 2015 to 2016, and coach on The Voice Australia from 2019 to 2024. In 2024 Sebastian was inducted into the South Australian Music Hall of Fame