The New Zealand international has reached an agreement with the London club to terminate his deal.
The 33-year-old, who has been with the Hammers for 11 years, is now ready to find a new club where he can play regular football.
"I have a lot of respect for the Club, and we've found an agreement that's good for everyone involved," Reid told West Ham TV.
"I came here in 2010 and have just enjoyed so much of playing for the Club. It's been a special time.
"I've always felt that the fans have supported me, and I've always tried to give my best for them. At the end of the day, when you're a footballer, that's what you enjoy the most, going out there and playing in front of a lot of fans.
"Over the years, that's also what I've enjoyed the most: playing for your fans, for your teammates, for your coaches and for your family.
"I just want to say 'thank you' and I wish you all the best for the future."
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