Tristan has been without a club since leaving Italian side
The 32-year-old has signed a permanent deal although the length and terms of the contract have not been released.
CEO Scott Duxbury told the official site, "I am delighted that we have been able to secure the services of Diego, who is an experienced striker and has already had a positive impact. He will provide extra competition in attack and the manager has been impressed with his attitude and ability in training.
"The focus for our squad is about quality not quantity and we are sure that Diego is a quality addition. This signing indicates that we are thinking only of the future progress of West Ham United."