Paul Anderson has become Bradford's seventh signing of the summer, agreeing terms on a two-year deal with the Bantams.
Bristol Rovers have signed former Northampton striker Billy Bodin on a short-term deal.
Buxton, who had 12 months remaining on his current deal, will now remain at Pride Park until the summer of 2015, with the option of a further year.
Rams boss Nigel Clough told the club's official website: "It's great news that Jake has signed a new contract with us.
"He has formed an excellent partnership with Richard Keogh in central defence and his new deal is a reward for his performances, as well as the way he has improved as a player to become an established Championship defender.
"His all-round game has improved since he joined us four years ago and he has an unbelievable desire to get his head on the ball in both penalty areas; he's just our kind of player."
Former Mansfield centre-half Buxton joined Derby in May 2009 on a free transfer from Burton, where he played under Clough.
The 28-year-old has scored five goals in 27 games this season, despite missing around two months of the campaign with a knee injury.
Buxton becomes the second player in eight days to sign a new deal at Pride Park after right-back Kieron Freeman put pen to paper on a new three-year contract last week.