Tamsin Olivia Egerton (born 26 November 1988) is an English actress and model best known for her roles as Chelsea Parker in the 2007 film St Trinian's, Holly Goodfellow in the 2005 film Keeping Mum and Guinevere in the 2011 TV series Camelot.
Egerton's other film credits include Sarah in Driving Lessons (2006) alongside Rupert Grint and Flora in Knife Edge (2008). She was to appear as Katrina in Eragon, but the scenes which she featured in were removed from the final cut.
Egerton starred as Princess Elenora in the TV children's series Sir Gadabout: The Worst Knight in the Land. In 2001, she starred as the "young" Morgaine in the TV miniseries The Mists of Avalon. She has also appeared on stage – in the RSC's musical version of The Secret Garden.
In 2009, she appeared in the St Trinian's sequel, St Trinian's II: The Legend of Fritton's Gold. In 2011, she appeared in Chalet Girl alongside Felicity Jones, Ed Westwick and Bill Nighy, playing "Georgie". In her second Arthurian role, Egerton plays Guinevere in the 2011 US Starz adaptation of the Arthurian legend, Camelot.