The central defender becomes the first player to put pen to paper on a new deal as manager Aidy Boothroyd begins to plan for next season following defeat in the npower League Two play-off final this time around.
"Kelvin epitomises the progress this club has made in the last 18 months," said Boothroyd.
"He has shown a fantastic attitude, he is a terrific leader both on and off the pitch and I am thrilled we have secured him.
"I was keen to get him signed early and to start our plans for next season by getting Kelvin signed up on a new two-year contract is fantastic news.
"We missed his influence when he was injured towards the end of the season and this is a big signing for us."
Langmead, 28, added: "It's kind of an unfinished business-type scenario.
"This season ended in disappointment but there has been a lot of progress made this season. I've really enjoyed this year, I've got back to really enjoying my football again and I want that to continue.
"I believe in the management here and the way the club is moving forward and I'm desperate to be a part of that.
"The season before last was a very difficult time for the club and to see the strides forward we've made and how far we've come is a credit to everyone involved."