Arsonists set fire to a sports pavilion in Cold Ashby.
The building in Stanford Road was set on fire sometime between 10.30pm and 10.45pm on Sunday, June 21.
Petrol was used to start the fire, causing structural damage to the building.
It is believed the fire may have been started by two people who were seen running away from the scene.
Witnesses, or anyone who knows anything about this incident, can call Northamptonshire Police on 101.
Alternatively, they can call the independent charity Crimestoppers anonymously on 0800 555 111 or use the anonymous online form at www.crimestoppers-uk.org.