The Moroccan Association for Human Rights, quoting a letter from the man's family, said US intelligence had transferred Younes Chekouri, who had been in Guantanamo "for years", on October 12.
"The family knows nothing about what has happened to him and has obtained no information from the Moroccan authorities," the group said.
It condemned such practices and called for an inquiry, involving the justice and interior ministers and the head of national security.
Of the nine others transferred from Guantanamo, five have been charged with membership of a terrorist organisation, two have been conditionally released and one is still in custody.
The ninth, Mohammad Ben Moujane, was handed over recently and is being prosecuted on terrorism charges.
The United States says three Moroccans are still being held at Guantanamo, while Moroccan Islamic groups insist there are four.

Copyright AFP (Agence France-Presse), 2006