The Englishman joins the Hammers on a four-year deal for an undisclosed fee after completing a medical.
Downing becomes Sam Allardyce’s fifth summer signing after the captures of Razvan Rat, Adrian, Danny Whitehead and former team-mate at Anfield, Andy Carroll.
Speaking to the official site Downing admitted he could not wait to get started. “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.
“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.”