The London club's website this morning announced the signing of the 22-year-old on a three-year contract from Danish Superliga club FC Midtjylland for an undisclosed fee.
Reid had a medical and met with new manager Avram Grant yesterday before completing formalities.
"I am looking forward to getting started," he told the club's website, while taking his first look around the Upton Park.
"My dream has been to come and play in the Premier League and I am happy that I am a West Ham player.
"It's been a mad couple of weeks and the last couple of days have been pretty hectic, so I am just happy to be here now. I am pretty decent fitness-wise. I had a game last Sunday so I am in pretty good nick."
Reid described his World Cup as "a life-changing experience" and said it was something he would remember forever.
"I want to bring the positives from that here on the pitch at West Ham. This club has great history and the league is one of the best in the world.
"I am looking forward to the challenge ahead."
Reports have the transfer fee agreed with Midtjylland between stg3 million ($NZ6.5 million) and stg4 million.
In recent weeks, Reid had been linked with possible moves to premiership sides Arsenal and to Blackburn Rovers.
He joins fellow All Whites Ryan Nelsen (Blackburn) and Chris Wood (West Bromwich Albion) in the premier league.
Reid, who is Grant's fifth close-season signing, scored the injury-time equaliser in New Zealand's 1-1 World Cup draw with Slovakia in the group stages in South Africa.
He was born in Auckland and moved to Denmark with his mother and Danish stepfather when he was 10,
He became a Danish citizen in 2006 and represented Denmark at under-19 and under-21 level.
Under a Fifa rule change, he became eligible for the All Whites because he had not played for Denmark at senior level.
