Grants arrival brings to end weeks of speculation that he was set to succeed Gianfranco Zola as Hammers boss after guiding relegated Portsmouth to the FA Cup final.
Speaking to the official site the new boss said, “I am proud and honoured to be the manager of West Ham. It will be an exciting challenge and I am ready to do my best.
“This is a fantastic club with great fans and a history that is respected around the world. I am already looking forward to getting to work with my players in July and preparing for the new season.”