Jenkinson, 23, returns to Upton Park after impressing on loan last campaign, but has also signed a new long-term deal at parent club Arsenal – reportedly for five years.
The one-time England cap featured in 32 games for the Hammers under Sam Allardyce last season, but has already expressed his excitement at joining new manager Slaven Bilic’s playing staff.
“I’m really happy to be back after last year,” he told West Ham’s official club website. “It was a good season for the Club and a good season for myself.
“To be honest I just wanted to come back here and I’m happy everything’s been sorted out. I’ve got another season here and I can’t wait.
“I’m a young player and I haven’t had too many experiences of different clubs, but I loved every minute of it here. Everything about it is fantastic – it’s a real family Club, the lads are great and the fans are great, so it ticks all the boxes you want when you join a football Club.
“I’ve got to come in, show what I can do and prove myself to a new manager. That keeps you on your toes and I’ve got to go and do that in pre-season. I’m looking forward to it.”
The Finnish-born defender becomes the Hammers’ fifth recruit of the summer.