The 29-year-old's previous deal was due to expire at the end of next season, but he will now stay with the club until 2017.
Speaking of Buxton, Derby head coach Steve McClaren said: "He has improved his game and his performances last season were excellent.
"His leadership both on and off the field in the dressing room and around the place has been spot on.
"We're very pleased that he is sticking around and you know what you are going to get day in day out from him."
Buxton played in all but one of Derby's league games last season, scoring two goals to help the Rams to the playoff final, where they were beaten by Queens Park Rangers.