The 22-year-old right-back has been linked with a host of Barclays Premier League clubs and decided to move across London for the 2014/15 campaign.
"I'm clearly seen as a big part of things here and hopefully I can prove my worth here because it is an important year for me," said Jenkinson.
"I was flattered to have a lot of options but deep down, if I was being honest with myself, West Ham was always the place I wanted to go to.
"It's one of them where I wanted to do the right professional thing and consider all the options, but in the back of my mind it was always West Ham and I'm just so glad it's all worked out how I wanted it to in the end.
"It will be brilliant to play here. I've got a lot of friends who are West Ham fans and I know how good the supporters are here.
"You get a great atmosphere here and the fans are great, so I just can't wait to get a West Ham shirt on and get on the pitch and play."
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