The aquatic mammal was spotted over the weekend in the Homosassa River in Citrus County,the US Fish and Wildlife Service (USFWS) and the Florida Fish and Wildlife Conservation Commission confirmed.
The area is about 80 miles north of Tampa.
"West Indian manatees are essential members of the ecosystems in which they inhabit and are protected under the Endangered Species Act and the Marine Mammal Protection Act," USFWS director Aurelia Skipwith said in a statement.
The manatee does not appear to be seriously injured, officials said.
It appears that the word was written in algae that had grown on the animal's back, not scratched into its skin.
Wildlife officials are looking for any information regarding the animal's harassment.
Harassing a manatee, a federal crime, is punishable by a 50,000 dollar (£37,000) fine and up to one year in prison.