An inmate at the US detention centre in Guantanamo Bay slashed his throat with a sharpened fingernail, US officials have confirmed.
The prisoner, described by his lawyer as an Algerian held for six years, required several stitches and spent a week under psychiatric observation.
US officials characterised the incident as an act of "self-harm" rather than a suicide attempt.
There are just over 300 prisoners still being held in the base in Cuba.
The latest incident reportedly took place last month in a shower.
US Navy Cmdr Andrew Haynes said there was "an impressive effusion of blood" but the prisoner was treated by guards and taken to the prison clinic.
Officials would give no details of the man but lawyer Zachary Katznelson said the inmate had been held without charge for nearly six years.
Cmdr Haynes said "self-harm" incidents were a tactic to discredit US forces.
There have been four suicides at the camp.
Two Saudis and a Yemeni prisoner were found hanged in June last year.
This May another Saudi was found not breathing in his cell and attempts to revive him failed.
Co-ordinated suicide attempts last May involving hoarded medicine led to tighter rules on the dispensation of pills.
On Wednesday the US Supreme Court is to hold a hearing on whether the inmates at Guantanamo Bay have the right to contest their detention in US civilian courts.