The 23-year-old defender was due to leave the League One club after Saturday's 3-2 defeat at home to Bradford City.
However, Spence's new deal at Stadium MK will expire on January 4.
"He's done wonderfully well since signing for us and we're very fortunate to have him until January. He is a typical MK Dons centre-back and they are hard to find."
Spence has made four appearances for MK Dons so far this season.