Citizen Sports has the number one fantasy American football app on Facebook and engages and unites fans across the world around their favourite sports team.
The acquisition aims to bolster Yahoo! Sports' reach, which was more than 39 million unique monthly users in February, according to Comscore.
Yahoo! Sports will be integrated into Citizen Sports' range of products and applications for football, hockey, soccer, baseball, racing, rugby, hockey and cricket.
In addition, Yahoo! Sports users will be able to broadcast their allegiances, create or join a conversation with friends and fans and cheer for their teams through Citizen Sports' applications.
The acquisition come as another of Yahoo!'s senior executives, Ash Patel, announced his departure from the business. Patel, who has been at the company for 14 years, follows top US ad sales executive and senior vice president Joanna Bradford out of Yahoo!.