The Republic of Ireland international joins the club until the end of the season with a view to a permanent move in the summer if the club avoid relegation.
Keane admits he is pleased with the chance to get some Premiership action under his belt after being frozen out at Spurs.
“I am really pleased to have signed and now I just can’t wait to start training and playing football,” Keane told the official site.
“I am ready to help West Ham United to stay in the Premier League, which is the most important thing for this club.
“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.”