The manatee, a species long considered at risk of extinction, has recovered in sufficient numbers to move from endangered to threatened status.
The US Fish and Wildlife Service is proposing to reclassify the protection status of the West Indian manatee, including a Florida subspecies listed as endangered since 1967.
The decision, announced at the Miami Seaquarium, reflects a finding that about 6300 manatees live off the Florida coast today, compared with roughly 1200 counted when aerial surveys began in the early 1990s.
Federal and state officials described the likely reclassification as good news for the whiskered "sea cow" popular among Floridians.
"Based on the best available scientific information, we believe the manatee is no longer in danger of extinction," Michael Oetker, deputy regional director for the US wildlife agency's southeast office, told a news conference on Thursday (local time).
Federal regulators were petitioned to downgrade the status of the manatee by the Pacific Legal Foundation acting on behalf of Save Crystal River Inc, a non-profit advocating for the rights of recreational water users in west central Florida.
Motorboat propellers and speedboats are a leading cause of death and injury to Florida's manatees.
The West Indian manatee, related to the African and Amazon species and to the dugong of Australia, grows to three metres and more than 450kg.
It has no natural predators, but is vulnerable to prolonged exposure to water temperatures below 20 C degrees.
The 2012 petition noted that US regulators in 2007 had recommended downgrading the manatee to threatened status, but never took action to reclassify the species.
"It's taken eight years and two lawsuits to get the government to follow up on its own experts' recommendation to reclassify the manatee," Pacific Legal Foundation lawyer Christina Martin said in a statement.
To be defined as endangered, a species must face imminent risk of extinction, wildlife regulators said, while being threatened indicates they could become endangered in the future.
If finalised, the classification change would not diminish protections for the manatee under the Endangered Species Act, wildlife officials said.
Manatees also are protected under another federal law for the conservation of marine mammals.
The range of the West Indian manatee extends into the US southeast, the Caribbean, Mexico, and Central and South America.
Regulators are seeking public comment on the proposed reclassification.
The agency has 12 months to adopt the change, or explain why it will not do so, said Martin of the Pacific Legal Foundation.