The family in Fuglie, southern Sweden, found the hefty gravestone, around 10 centimetres thick, tucked in under the floor.
"I was quite surprised. And my wife is still getting over the shock. It's the last thing I could imagine finding under my floorboards," Gert Nilsson told 'The Local'.
"We have no idea how we're going to get it out. It's huge - it must weigh 300 kilogrammes," Nilsson said.
Nilsson explained that his house is "a pretty old one" and that the floors are directly on the ground.
The stone had the names of two men and a woman who died in 1843, 1851, and 1884 respectively.