The author of an internationally bestselling memoir about life in Guantánamo Bay has been released after enduring 14 years and severe torture at the US detention center.

Mohamedou Ould Slahi, whose 2015 Guantánamo Diary memoir yielded a new wave of global outrage over Guantánamo Bay, is now free in his native Mauritania, his US attorney Nancy Hollander confirmed.

Hollander said she has yet to speak directly with Slahi and could not yet vouch for his health, but the American Civil Liberties Union released a statement it attributed to the detainee-turned-diarist.

"I feel grateful and indebted to the people who have stood by me. I have come to learn that goodness is transnational, transcultural, and trans-ethnic. I'm thrilled to reunite with my family," the statement quoted Slahi saying.

The Pentagon subsequently announced Slahi's transfer, which has brought the Guantánamo population down to 60 men.