The prisoner - identified as Inayatullah - had been detained in the prison since 2007.
The US military said in a statement the prisoner was found "unresponsive and not breathing".
"After extensive life-saving measures had been exhausted, the detainee was pronounced dead by a physician," the statement said.
This was reportedly the sixth prisoner suicide case since the detention centre opened in January 2002.
(Published 20 May 2011, 03:51 IST)