Jenkinson, 22, joined Arsenal from Charlton in 2011 and earned his only England cap against Sweden 18 months later.
"I'm clearly seen as a big part of things here and hopefully I can prove my worth because it is an important year for me," said Jenkinson, who joins Mauro Zarate, Enner Valencia, Cheikhou Kouyate, Aaron Cresswell and Diego Poyet in a new-look West Ham squad.
"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."
- Carl Jenkinson
- Mauro Zarate