The Hammers have been without a shirt sponsor since the collapse of Travel and Leisure firm XL two months ago and have been playing their matches wearing a white squad number patch on the front of their shirts.
The deal is believed to be worth £1.9 million and will run until the end of next season.
The club have also announced that child replica shirts will not bear the logo of the betting company but will instead have the logo of The Bobby Moore fund for Cancer Research
Hammers CEO Scott Duxbury told the official site, "This is a great deal for West Ham United and I am looking forward to a successful partnership with SBOBET. Finding a new shirt sponsor usually takes up to six months but a lot of effort and hard work has gone into this process. I would also like to thank the fans for their understanding and patience during this time."