His arrest happened following a social media campaign calling for justice for the old man who was tortured by the police constable Ali Hassan in Ghotki district.
The victim has been identified as octogenarian Gokal Das.
The constable has been arrested fort torturing and injuring Das on Friday.
Vinod Kumar, Gokal Das's grandson, registered an FIR against the policeman for assaulting the elderly man, a senior police official said.
Inspector General Sindh A.D. Khawaja ordered an inquiry after picture of Das with torture marks went viral on social media.
Rights activists including Sudhar Sabha president Amarnath Randhawa demanded an exemplary punishment for the policeman.