The actor - who won a BAFTA last year for his role as foul-mouthed political spin doctor Malcolm Tucker in BBC Two comedy The Thick of It - will play eccentric head of news Randall Brown when the 1950s newsroom drama returns in 2012.
Capaldi has also recently portrayed civil servant John Frobisher in Torchwood: Children of Earth and appeared in a supporting role as Frank Ryland in the first series of Jimmy McGovern's acclaimed courtroom drama Accused.
He is currently playing Professor Marcus in a stage version of Ealing comedy The Ladykillers adapted by The IT Crowd creator Graham Linehan.
Romola Garai, Ben Whishaw and Dominic West formed the core cast in The Hour's first series, which also featured appearances from Anna Chancellor, Juliet Stevenson and former Torchwood actor Burn Gorman.
Whishaw has since been cast as gadget master Q in the new James Bond movie Skyfall, which is due out next October.
Garai has promised that the new series of The Hour, to be set around nine months after the end of series one, may reference world events including the launch of Sputnik and the Notting Hill race riots.
Filming on series two of The Hour - created and written by Abi Morgan - began this week in London.
> Peter Capaldi: 'I'm not Malcolm Tucker'
Watch a clip from series one of The Hour below: