Sio Agafili, 45, was meant to have been released in December 2015, but the error was only picked up on by a judge when he appeared in the Supreme Court last week.
"He's been imprisoned unlawfully," said Muriel Lui, who was appointed as Agafili's lawyer by the court when the error was spotted.
Lui said she was concerned there could be plenty more prisoners like Agafili languishing in Samoa's much-maligned prison system because of basic errors.
"I've heard of other cases of a similar nature," she said. "This is not the first of this type."
