The combination will create a company that offers delivery service in 500 U.S. cities with connections to 20,000 local takeout restaurants.
The two companies in 2012 referred $875 million in business to local restaurants, the service companies said in a statement.
The services together are expected to generate $100 million in revenue within a year of the closing.
"GrubHub and Seamless share a common goal to generate more business for local takeout restaurants while providing the best possible service to diners. By combining our complementary restaurant and diner networks, we are well positioned for continued growth in a massive market, " said Matt Maloney, GrubHub co-founder and chief executive officer.
"Both companies also share a strong commitment to provide world-class service to restaurants, diners and corporate clients. This merger is an opportunity to glean the best from each platform and improve upon what we bring to all of our partners," said Seamless CEO Jonathan Zabusky.
The value of the deal was not released. After it closes, Maloney will keep his CEO position, while Zabusky will serve as president. Brian McAndrews, an independent director on the Seamless board, will become the company board's chairman.
The companies also did not announce the name of the new combined company.
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