Moncur has been at Upton Park for the last nine years and would have been entitled to a testimonial next year, but he has decided instead to take a complete year away from the game.
"I have decided to retire. I made the decision two weeks ago really, but I have not told anyone except my family," he told the clubs official website.
"It is not one of those situations you take lightly, but I took my time, had a good think about it, spoke to my family, took some holiday, and decided I had had enough.
"I always wanted to finish at West Ham and although it is a little bit disheartening for me the way it ended it was a great time for me with great fans and it is a great club and hopefully they are going to crack on."