Vandals broke into Oxford United's Kassam Stadium and burnt the initials of rivals Swindon Town into the pitch.
Police have appealed for witnesses to the incident, which happened at around 5.36am on Saturday, August 13. The club said officers had begun an investigation and have CCTV footage of the people involved in defacing the ground with the letters STFC.
PC Paul Phillips said: "We are studying CCTV and have images of persons on the pitch at 5.36am. We are working closely with the club and Ozone security with a view to identifying these people. Anyone with information is asked to contact police on 08458 505505."
United's game with Bradford City went ahead despite the vandalism, and ended in a 1-1 draw. Oxford United take on Swindon in the lunchtime fixture on Sunday.