Maryland Department of Natural Resources spokeswoman Heather Lynch says an angler was crabbing when he saw the giant snakehead in the water near Fells Point.
The department's spokesman John Surrick says the man saw the fish floating in distress on the surface, and it obviously had been dumped from an aquarium.
It's the first sign of the predator fish in the Chesapeake Bay.
Surrick said snakeheads cannot survive in salt water and does not pose a threat to the bay.
The notorious Chinese fish are known for being voracious predators that can slither out of water — from one body of water to another.
The announcement came two days after state officials poisoned a pond in nearby Anne Arundel County to kill snakeheads that had been found breeding in the pond.
The state decided to eliminate the snakeheads out of fear what the Chinese fish might do to the ecosystem if they escaped from the pond and established a population in Maryland. Snakeheads sit at the top of the food chain in their native Yangtze River, and would pose a threat to native fish species.