Chantae Kelly, aged 13, left her school in the town at 3.10pm and failed to return to her home address.
Chantae is black, 5ft 2in, with a medium build and shoulder-length, curly hair, which she often wears up. She was last seen wearing a black jacket, blue denim jeans, a navy T-shirt and trainers.
A police spokeswoman said: "We would like to appeal for Chantae to come forward and contact the police so that we can check she is safe and well."
If anyone has seen her or knows of her whereabouts, please contact Northamptonshire Police on 101