The Hammers have agreed a deal which takes the Republic of Ireland striker initially on loan until the end of the season with a view to making a ?6million deal permanent for the next two-and-a-half years, as long as they maintain their top flight status.
Keane told his new club's official website: "I am ready to help West Ham stay in the Premier League, which is the most important thing for this club."
Keane passed a medical on Saturday night and put pen to paper on a contract after watching Sunday's 3-2 defeat of Nottingham Forest, which puts the Hammers in the last 16 of the FA Cup. He could make his debut in Wednesday's match at Blackpool.
He added: "I am really pleased to have signed and now I just can't wait to start training and playing football. I feel as fit as I ever have done in my life and I feel I have plenty left to offer at the highest level for at least the next four or five years.
"I want to help anywhere I can help the football club, whether that is in training, in the dressing room or on the pitch on a matchday. We have some good players here and I am sure we can stay up. When I went on loan to Celtic I scored 17 goals in 19 games and if I can do even half as well as I did there, then I'll be happy."
Source: PA
Source: PA