The New York-based organization described the arrest of the Sahrawi activist as "a human rights violation by the polisario."
He was arrested "upon his return from" a visit to the Sahara for having "publicly announced his support" for the Moroccan autonomy proposal, it noted.
HRW added that the polisario had finally announced Salma's release on October 6.
"At this writing he remains under Polisario auspices while the United Nations High Commissioner for Refugees work to resettle him in a place of his choosing," the American NGO said.