The 25-year-old, has built up a reputation with Cardiff City in the Championship and as a fully fledged Welsh international and was always likely to make the step up to the Premiership this summer.
Gabbidon is joined at West Ham by fellow Wales international James Collins who he became friends with during their time together at Ninian Park.
Gabbidon said: "The Championship is a very good league and I felt that I've learned a lot there, but every player wants to play against the top teams in the Premier League and I'm just delighted to have that chance.
"You watch the games and the great players on TV and it makes you want to be up there. I can't wait to test myself against some of the best players in the world and hopefully I can show people what I can do.
"A move to the Premiership with a massive club like West Ham United is just a fantastic opportunity and now I hope to take advantage."
While Gabbidon could walk into the Hammers' first team, Collins could find himself in the reserves until he can establish himself at Upton Park with the club brimming with central-defensive talent in Anton Ferdinand and Elliot Ward, who were both solid during their promotion campaign in the Championship.
Gabbidon added: "Neither me or James Collins are taking anything for granted. It's going to be very tough to get into the side.
"There are some excellent players at West Ham United and we're going to be part of a very strong squad.
"The players that won promotion last season did extremely well and it's up to James and myself to prove our worth to Alan Pardew."
Pardew said: "Gabbidon has already shown at international level that he has what it takes to perform at the highest level.
"He is already a key player for the Welsh national team and has shown a lot of maturity in his performances.
"James is an excellent prospect. He is probably lacking experience as he joins the club, but do not underestimate his potential.
"At 21 he has already played for the Welsh national side, and with the signing of James and Danny we now have five of the best young centre-backs in the country."