Theodore Peter James Kinnaird Taptiklis (born 16 December 1984) is an English actor and producer. He gained recognition for playing Tobias Eaton in The Divergent Series (2014–2016). He has starred in the horror films Underworld: Awakening (2012) and Underworld: Blood Wars (2016), the science fiction film Archive (2020), and the dark comedy The Monkey (2025) in a dual role as a pair of twin brothers.

In television, he appeared in the short-lived CBS crime drama series Golden Boy (2013), the romance series The Time Traveler's Wife (2022), and the dark comedy series The White Lotus (2022), which earned him a nomination for a Primetime Emmy Award. Since 2024, he has played the lead role in Guy Ritchie's action comedy series The Gentlemen.

Theodore Peter James Kinnaird Taptiklis was born on 16 December 1984 in High Wycombe, England. He is the son of Philip Taptiklis, a business consultant from New Zealand, and Jane (née Martin), who worked for the National Health Service. His paternal grandfather was a Greek refugee who fled Greece for Syria during World War II. He is also of part Scottish descent. He has two older brothers and two older sisters.

James was raised in Askett. He attended Aylesbury Grammar School, and earned his undergraduate degree in philosophy from the University of Nottingham. He trained at the Bristol Old Vic Theatre School.

In 2010 James made his television debut in two episodes of A Passionate Woman. The same year he played Turkish diplomat Kemal Pamuk in an episode of the first season of Downton Abbey. His film credits include You Will Meet a Tall Dark Stranger (2010). James was cast in Stranger while still in his final year at drama school. He was named a "Star of Tomorrow" in 2009 by Screen International.

In 2011, James starred in the short-lived horror series Bedlam, playing the lead role of Jed Harper. That same year, he also played James, an obnoxious night club rep, in the British comedy The Inbetweeners Movie (2011). In 2012 James appeared in the miniseries Room at the Top as Jack Wales and ITV's Case Sensitive as Aiden Harper. He was in the films The Domino Effect (2012) and Underworld: Awakening (2012), playing the role of David in the latter opposite Kate Beckinsale. He reprised his character David in the fifth Underworld film, as the film's male lead. James starred as Detective Walter William Clark Jr. in the American crime-drama series Golden Boy (2013), which aired for one season.

James' breakthrough role was his portrayal of Tobias "Four" Eaton in Divergent (2014), a film adaptation of Veronica Roth's novel of the same name. He reprised the character in the sequel, The Divergent Series: Insurgent (2015). He returned in one more Divergent film, The Divergent Series: Allegiant (2016). James performed his own stunts in the film series.

James starred alongside Amber Heard, Billy Bob Thornton and Jim Sturgess in the film adaptation of London Fields, and in the indie drama The Benefactor (2015), with Richard Gere and Dakota Fanning. James executive produced and starred as Michael in Backstabbing for Beginners (2018). He starred as Will Younger in the Netflix film How It Ends (2018). In 2019 James executive produced and played Sidney Parker in the British series Sanditon as a male lead. It is an eight-part period-drama adaptation of Jane Austen's final novel, written only months before her death in 1817. James did not reprise his role for the series' second and third seasons. He then executive produced and starred in the film Lying and Stealing. James launched his own film and TV production company, named Untapped, that same year.