Toshiko "Tosh" Sato (佐藤 とし子, Satō Toshiko) is a fictional character from the television series Doctor Who and its spin-off Torchwood, played by Naoko Mori. After a one-off appearance in the Doctor Who episode "Aliens of London" (2005), Toshiko is re-introduced as a series regular in the Torchwood 2006 premiere episode "Everything Changes". The character appears in every episode of the show's first two series as well as Expanded Universe material including Torchwood novels, audiobooks and comic strips.