The bronze helmet was found with bone fragments, and had been used to hold human remains after a cremation, Canterbury Archaeological Trust said.
The finder contacted archaeologists because he was confident he had made a significant discovery, the trust said.
University of Kent experts have found it dates back to the 1st Century BC.
Andrew Richardson, finds manager at the trust, said the person who found the helmet wanted to remain anonymous.
A brooch that would have fastened a bag holding the cremated bone was also unearthed, he said.
Julia Farley, Iron Age curator at the British Museum, said it was one of a handful of Iron Age helmets found in Britain.
She said it was not unusual to bury cremated remains in a bag fastened with a brooch in late Iron Age Kent.
But she said: “No other cremation has ever been found accompanied by a helmet.”