After buying theatre and ticketing website in January 2012, the Time Out Group has sold it to New York-based TheatreMania.com.
Both Whatsonstage and Theatremania run news, reviews, and ticketing websites as well as organising theatre trips for its members.
A joint statement from the two organisations described how they "share the mission of providing patrons with easy access to theatre tickets and information through [our] respective websites, digital newsletters and apps."
Whatsonstage.com's founding editor and managing director Terri Paddock said that the two groups would "create the first dedicated transatlantic digital theatre media group serving the world's two great theatre capitals, London and New York, and markets throughout the United States and United Kingdom."
Gretchen Shugart, TheatreMania's chief executive, also added: "TheaterMania has enjoyed a close friendship with WhatsonStage.com for many years and is pleased to finally unite our two companies. We can better serve global audiences and productions by leveraging each other's strengths in our respective markets."