First Artist Corporation, which bought Dewynters in 2006 for £15.5m, is to pay up to $18.9m (£9.9m) for SpotCo, depending on the extent its performance targets are met.
Dewynters, which was founded in 1976, has promoted musicals including 'Cats', 'Phantom of the Opera' and 'The Lion King' and has had a long relationship with musical composer Andrew Lloyd Webber.
The two agencies have collaborated on a number of worldwide campaigns in the past decade, including one for 'Chicago' that reached 28 countries as well as recent Broadway to West End transfer 'Avenue Q'.
Each agency will continue to work independently with other agencies and clients under their own identities.
The other major New York theatrical promotions agency, Serino Coyne, was bought by Omnicom in 2003.
Drew Hodges, the founder, president and director of creative services at SpotCo, told The New York Times that musicals have become global. "A show could go from London to New York to Australia to Berlin to Toronto."
Last week the founder of live entertainment company Cirque du Soleil sold a 20% stake in the company to Dubai World, the investment company that owns the QE2. The sum paid was not disclosed.