Erin Moran has died at the age of 56.
The actress was best known for playing Joanie in 1970s sitcom Happy Days.
Police said an emergency team in Indiana responded to a call on Saturday afternoon and found her dead.
The cause of death is not yet known.
Erin Moran also starred in the show’s spin-off series Joanie Loves Chachi, which began in 1982. She played Joanie Cunningham, the younger sister of lead character Richie.
Richie was played by Ron Howard, who has been among those posting the first tributes online.
Erin Moran joined the Happy Days cast in 1974 when she was 13.
She also made appearances in Murder She Wrote and The Waltons.
In recent years, Erin Moran was reported to be living in an Indiana trailer park.
In 2010, Erin Moran joined various Happy Days co-stars in filing a lawsuit against CBS, claiming they were owed money from merchandising revenues.
The suit was settled in 2012, with the actors each receiving a payout of $65,000. They originally asked for $10 million.