Sigrid Madeline Thornton is an Australian film and television actress. Her television work includes Prisoner (1979–80), All the Rivers Run (1983), SeaChange (1998–2019) and Wentworth (2016–2018). She also starred in the American Western series Paradise (1988–91). Her film appearances include Snapshot (1979), The Man from Snowy River (1982), Street Hero (1984) and Face to Face (2011). She won the AACTA Award for Best Guest or Supporting Actress in a Television Drama for the 2015 miniseries Peter Allen: Not the Boy Next Door.

Prisoner is an Australian television soap opera, which broadcast on Network Ten from February 27 (Melbourne) February 26 (Sydney) 1979 to December 1986 (Melbourne), though the series finale would not screen until September 1987 in Sydney, where it aired as a three-hour film that was split into three one-hour episodes at the much-later time-slot of 10:30 p.m., running eight seasons and 692 episodes.