The 29-year-old, who moved to Anfield from Aston Villa in 2011, has penned a four-year deal at Upton Park.
Downing links up with former Liverpool team-mates Andy Carroll and Joe Cole at West Ham, who have also added Romania international defender Razvan Rat, goalkeeper Adrian and 19-year-old midfielder Danny Whitehead to their ranks during the close-season.
The England international is relishing the prospect of a fresh start under Sam Allardyce and has declared himself fit to be considered for West Ham's Premier League opener at home against Cardiff City on Saturday.
"I'm very excited. It's a great opportunity for me being here, it's a great club and I'm really excited to get going," Downing told the club's official website.
"I'm ready and I'm fit for the first game against Cardiff City. I've played a decent few games for Liverpool in pre-season, so there will be no problems on that front.
"It's all happened quickly, but I'm really excited and hoping to play a part on Saturday."
Downing scored five goals in all competitions as Liverpool finished seventh last season and made 91 appearances in total for the club.
Get the best features, fun and footballing frolics straight to your inbox every week.
Thank you for signing up to Four Four Two. You will receive a verification email shortly.
There was a problem. Please refresh the page and try again.