A man in his 50s has died after sustaining a critical head injury in Brisbane's CBD.

Queensland Ambulance Service attended a location on Mary Street about 4am after reports of a serious incident.

Four units, including the High Acuity Response Unit and a Critical Care Paramedic, treated the man but he died at the scene.

Police said they believed the man to be alone at the time of the incident but did not have further information as to how the incident happened.

The death is being treated as non-suspicious.