The production, which will be directed by Howard Davies, runs from 18 October to 24 November 2012.
Gatiss, who rose to fame as a member of comedy group The League of Gentlemen, has recently been on stage at the Donmar Warehouse in The Recruiting Officer, at the National Theatre in Season's Greetings and All About My Mother at the Old Vic.
His myriad TV credits include playing Mycroft Holmes in the BBC's Sherlock, and major roles in The Crimson Petal and The White, Being Human, Worried About The Boy and George Gently.
55 Days charts the political upheaval of the mid-17th century. In these dangerous and dramatic times, in a country exhausted by Civil War, the great men of the day were trying to think the unthinkable – to create a country without a king. With Charles I refusing to compromise, Oliver Cromwell struggled to invent a political future for his country as he presided over the death of medieval England and the birth of the modern state.
The play runs as part of Hampstead Theatre's autumn season which also includes David Hare's The Judas Kiss directed by Neil Armfield and starring Rupert Everett and Freddie Fox, and the premiere of Sarah Wooley's Old Money directed by Robin Lefevre and starring Maureen Lipman and Tracy Ann Oberman.
Running from 15 November 2012 to 12 January 2013 (previews from 8 November), the play centres on influential WW1 poet Edward Thomas and his friendship with American poet Robert Frost.