The 29-year-old has signed a four-year contract and reports suggest the Hammers have paid around £5million for the winger.
“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 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.”
