Firefighters from Ossett, Wakefield and Dewsbury were called to Dudfleet Mill at Horbury Junction at around 6.30pm on Tuesday.
They discovered a pallet and rubbish burning inside the mill.
Crews remained at the scene until around 7.30pm.
A police spokeswoman said: “Police were called to reports of criminal damage at a disused building on Dudfleet Yard at around 6.40pm yesterday.
“Officers were called by the fire service after a fire had been lit inside the warehouse.
“Enquiries are ongoing into the incident. Anyone with information is asked to contact police on 101 quoting crime reference number 13170094620.