The 24-year-old Irish actress is said to have beaten over 100 hopefuls to land the part, opposite Taron Egerton, reported Deadline.
The Otto Bathurst-directed film will go into production next summer right after Egerton wraps the sequel to "Kingsman: The Secret Service".
Others who were shortlist for Maid Marian's role included Dutch actress Gaite Jansen ('Supernova'), Lucy Fry ('Vampire Academy') and British actress Gugu Mbatha-Raw ('Belle').
Hewson made her feature film debut in 2005 with "Lost and Found". She'll stars alongside Tom Hanks and Mark Rylance in the Steven Spielberg directed "Bridge Of Spies".