Isabella Thornton, formerly Lady Isabella of Gisborne, is the secondary antagonist of the third season of the 2006-2009 TV Series Robin Hood, the sister of Guy of Gisborne and the new Sheriff of Notthingam after Vaisey of Nottingham's alleged death until being captured after a popular revolt led by Robin Hood.

She is introduced in episode 5 of series 3, having recently escaped her husband, and Robin saves her from being taken back. She begins a romantic relationship with Robin and begins to flirt with Prince John in order to compete for his favor with her brother. While initially helping our heroes in achieving their goals, she later betrays Robin, after he refuses to live with her and start a fresh life somewhere else, joining Prince John's side. After Gisborne is outlawed and imprisoned, he appoints Isabella as the new Sheriff of Nottingham.

Isabella kills Robin by lightly cutting his neck with a timed poison. After doing so, Robin kills Isabella and Vaisey by exploding the byzantine barrels. A few hours later, Robin dies in Sherwood Forest.