The 45-year-old woman was kayaking just off Big Pine Key in Florida when it happened.
She says a fish jumped out of the water and hit her in the back.
She was taken to the hospital with a punctured lung and broken rib.
Experts are shocked, saying they don't know what kind of sea creature might have acted that way.
"It's not a report that I've seen in two years since I've been here," said Coast Guard Lt. Anna Dixon.
The woman is expected to make a full recovery.