Yossi was top scorer for Santander last season, quite an achievement for a player who usually plays an advanced, creative midfield player. The four-year-deal is subject to a work permit, although the club do not anticipate any problems because Yossi is an established and current international.
Hammers managing director, Paul Aldridge, said: "We are delighted to have signed such a highly-rated player who was being chased by a number of other leading Premiership clubs. I am sure he will become a huge favourite with our fans."
Yossi didn't take long to agree terms with West Ham United, and afterwards revealed how much he is looking forward to his Upton Park debut.
He said: "I am absolutely thrilled to be joining such a big and famous club as West Ham United and can't wait to play in the Premiership.
"I know other clubs were interested in me, but I was very impressed when I met the people at West Ham. I also think my qualities will suit their style of play.
"I also spoke with my wife and family, and former West Ham star Eyal Berkovic, who told me not to hesitate in signing for them. Eyal and I are good friends and we have played together for the national team and he always spoke very highly of the club and its supporters.
"I know something of the history and tradition of the club and their supporters and saw their play-off final and how they celebrated afterwards. I'm looking forward to playing for West Ham United."
Hammers boss Alan Pardew added: "We're delighted to have secured the signature of Yossi and I'm sure he'll prove to be a great asset to our squad.
"He's a creative midfield player, who brings an educated discipline to the attack. But he's also a diligent player, who hardly misses a game and has improved year on year La Liga.
"He's a player with flair, who'll be able to open the door for us when teams sit deep. We needed a player like that and at 25-years-old he is a great signing for the club."