Under the long-term agreement, Marathonbet will also take on the title sponsorship of the club’s annual home pre-season friendly, which will now be known as the Marathonbet Cup.
Marathonbet is already familiar with sponsorship in the Premier League after it struck a two-year deal with Fulham last season.
However, West Ham’s fellow London club was relegated from the Premier League last season, ending a 13-year stay in England’s top division.
Graham Luke, chief executive of Marathonbet, said: “The future of West Ham United looks very bright, with the club preparing to move to the Olympic Stadium.
“The club's ambitions echo our own as we look to grow and expand into new territories. This global agreement is a key vehicle in helping us reach our strategic goals.”
West Ham finished the 2013-14 Premier League season in 13th place.