Tristan has spent the last two-weeks on trial with the Hammers.
The 32-year-old Spaniard was released by Italian side Livorno at the end of last season.
He told the club’s official website: “I am really happy to be at West Ham. They are a big club and London is a great city. I want to show the fans that I can score goals and help the team go up the league. I am excited to be here.
“Zola has helped me a lot and I want to say thanks by scoring lots of goals and doing well for the club.
“I am very grateful to the manager for giving me the opportunity to play here.
“I have been really impressed from the moment I arrived at the training ground. The coaching staff and the players have all helped me to settle in.
“I had a lot of offers to go to other countries and play for a lot of money but I feel that I still have lots to offer and I want to show West Ham fans that I can still play at the highest level. That is why I am here.”
He added: “English football is very similar to Spanish in that it is very quick and technical. There is more of an emphasis on attacking football so I will have more chances to score goals.
“In Italy, it was more defensive and not easy for forwards to get scoring opportunities. West Ham play the right way and that should help me to do well.
“I have always been impressed by English football whenever I have played here. The stadiums and the fans are great and it was always a dream of mine to come here and play.
“Now my dream is going to become reality. I am at a great club and I want to see if I can help West Ham get into Europe and play at that level.”