Dina Mitchell told ABC News that the Flex 2 began to combust on her wrist while she was reading a book.
“It burned the heck out of my arm,” she said.
Fitbit has said it is “extremely concerned” and is now looking into the issue, though it sees “no reason” for people to stop wearing the Flex 2.
Ms Mitchell said she quickly removed the tracker from her arm and threw it on the floor.
A doctor had to take small pieces of plastic and rubber out of the wound following the incident, she claimed.
“We are not aware of any other complaints of this nature and see no reason for people to stop wearing their Flex 2,” Fitbit said in a statement.
“We will share additional information as we are able.”
The batteries in many electronic devices are sometimes susceptible to overheating and have been known to catch fire or explode in other cases.
Last year, Samsung had to recall its Galaxy Note 7 smartphones after the handsets were found to be prone to combusting.