TED, MediaCom's entertainment division, will handle strategy, planning and buying for TV, cinema and online. It won the account without a pitch.
AKA, Stage Entertainment's incumbent full-service agency, will continue to oversee planning and buying for print, outdoor and radio.
The company is gearing up for a push across all media from September, kicking off with cinema advertising for the musical Sister Act.
The Dutch-owned group owns and licenses more than 70 productions worldwide and owns or runs theatres in 11 countries, including the UK, the Netherlands, Germany, Spain, Italy, Russia and the US.
Chris Harper, the commercial director at Stage Entertainment, said: "MediaCom's knowledge and passion for our work has been very impressive and we can't wait to get started."