The Fos Williams Medal has been awarded since 1981 to the best Australian rules football player from South Australia during Inter-State or Inter-Competition matches. The medal is named in honour of legendary South Australian National Football League (SANFL) player and coach Fos Williams.
Originally the award was made in respect of State of Origin matches. Since that competition folded in 1999, the medal has been awarded during games where the SANFL plays other state competitions in State League representative matches (e.g. WAFL v SANFL). The award is usually voted on by a panel selected by the SANFL from significant football identities present at the match, and may include past players, coaches and journalists.