The New Zealand international had been heavily linked to Arsenal and Tottenham, with his previous deal expiring at the end of the season.
But Reid, 26, said he was happy at the Upton Park - and feeling valued was one of the key reasons he decided to re-sign.
"It’s one of those things that took a bit of time, but we got there in the end and I’m very happy to be here for the next part of my career. I’m looking forward to it," he said in an official West Ham statement.
"At the end of the day, for myself I feel comfortable here. I was speaking with the joint-chairman David Sullivan and everybody and felt like I was valued here. He was clearly very committed to getting the deal done and keeping me at the club, so I’d like to thank him for that and I’m delighted to sign for the next six years.
"My family are happy living here too. We’ve been part of the club living in London, so we weighed all the factors up and I’m delighted to be staying here."
