The injured man was sitting on a balcony at his home when "lava spatter" - projectile molten rock - landed on him.
"It hit him on the shin and shattered everything there down on his leg," a spokeswoman for the county mayor said.
Lava spatters can weigh "as much a refrigerator", she told Reuters.
The Kilauea volcano on Hawaii's Big Island erupted at the beginning of May, and the situation for residents has steadily been worsening.