Tokyo Electric Power Company spokesperson Shogo Fukuda says the man in his 50s was rushed to the hospital but his heart stopped beating.
Other details were not immediately available. Tepco says the worker was digging near storage containers when he was buried on Friday.
Three reactors at Fukushima Dai-ichi melted down after the 11 March 2011, earthquake and tsunami damaged the plant's cooling system.
Thousands of workers, wearing masks and suits to guard against radiation, have been working to clean up the plant.
Workers have collapsed from heat and ailments. But this is the first major accident involving a plant employee since the tsunami killed two workers.