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.
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