The storm, named Ana, was upgraded from a subtropical storm as it became more organized and gained strength with winds at 50 mph, said James Franklin of the National Hurricane Center in Miami.
At 11 a.m., Ana was centered about 460 miles east-southeast of Bermuda and moving east near 16 mph.
The Atlantic hurricane season runs from June 1 to Nov. 30. Ana is the first tropical storm to form in the month of April, though storms have developed as early as January, Franklin said.
Experts have predicted a busier-than-normal hurricane season, but Franklin said Ana's development doesn't mean that more tropical storms than usual will develop this year.
"I would consider it a fluke event and I wouldn't read anything into it," he said.
Ana was only the second subtropical storm to form in April since forecasters began tracking hurricanes in 1871. The only other known April subtropical system formed in 1992, forecasters said.