Cooper, who joined the npower League Two club at the start of November, will now stay with the Spireites until January 5, 2013.
The 21-year-old centre-half has made five appearances for Chesterfield, who have won on four occasions when he has played.
Cooper opened the scoring against Tranmere in the FA Cup at the weekend before the League One leaders hit back to win 2-1.
The Hull-born defender remains available to Spireites boss Paul Cook for the forthcoming league matches against Port Vale, Burton, Southend, York, Morecambe, Accrington and Wycombe.
- Sports & Recreation