The victim Isakki Raja from Tirunelveli was talking over phone when he accidentally touched a wire, without realising it was a high voltage electricity line, police said.
Under the impact, the thumb on his right hand was severed from the middle and fell on the floor, they said.
On hearing his screams, room mates and neighbours rushed to the spot and took him to a nearby private hospital, where he is undergoing treatment, police said.
Raja had come for an interview and was staying in a rented room in the first floor of a house in Peelamedu area, police said.