The couple reportedly met on the set of the 2013 film Kill Your Darlings.
Image: Daniel Radcliffe with his co-stars Rupert Grint and Emma Watson in Harry Potter 20th Anniversary: Return To Hogwarts Pic: PA Photo/Warner Media
Radcliffe, who played the titular character in the film series between 2001 and 2011, divides his time between the UK and the US.
He has also starred in 2012 horror film The Woman In Black and 2016 surreal comedy Swiss Army Man.
Ms Darke, who was born in Michigan, is also an actor who is best known for her role in 2015 series Good Girl Revolt, and more recently appeared in Prime Video series The Marvelous Mrs Maisel.