The Hammers said they chose the group for its "close links to the community".
Vice-chairman Karren Brady said the deal demonstrated the club had kept its promise to regenerate two areas of east London through its stadium move.
The developer plans to build new homes on the site as well as retail and leisure facilities by 2018.
It has also been in talks with the family of former West Ham and England captain Bobby Moore about the possibility of placing a landscaped garden named in his honour at the centre of the site, as well as a statue of him.
In a statement, Moore's daughter Roberta Moore said: "I have always believed that there should be some form of permanent West Ham United presence at the Boleyn Ground site after the team leave and I'm really pleased that the centre point of the development is planned to be named in honour of my father."
West Ham have agreed to sell Upton Park to London developer the Galliard Group once they have completed their move to the Olympic Stadium in 2016.