The 29-year-old New Zealand international has committed his future to the Hammers until 2023.
Reid, who joined West Ham from FC Midtjylland in 2010, told the club's website: "I'm very happy.
"I'm pleased we've got it sorted so I'm obviously looking forward to the seasons ahead. It's going to be a big amount of time I've spent at the club and I'm looking forward to it.
"I've been here a long time now. I think with the business we've done over this summer and the players we had here before, I think we can go on to have a good season and seasons ahead. It's all about trying to get better.
"I enjoy being here and my family does as well so I'm very happy I'm able to stay here."
Reid has so far made 204 appearances for West Ham and scored 10 goals.