Gonzalo Montoya Jiménez, an inmate at a jail in the Asturias region, regained consciousness just hours before an autopsy was due to be performed.
His body is said by his family to have been marked in preparation for the post-mortem examination.
He is now under observation in intensive care at an Oviedo hospital.
Family members told local newspaper La Voz de Asturias that Montoya "had autopsy markings on him, ready to be opened up".
