"Some 85 people have died since late Saturday night," Dr Seemin Jamali, joint executive director of the Jinnah Postgraduate Medical Centre (JPMC) was quoted as saying.
Most of the victims were men in their 50s.
Saturday was the hottest day of this year in Karachi, where the mercury had shot to 45 degree Celsius. The maximum temperature of 48 degree Celsius was recorded in three districts of Sindh - Jacobabad, Larkana and Sukkur - on Saturday, which slid down to 41 on Sunday.
No respite from heat wave is expected soon and temperature likely to go up to 44C.
The highest temperature Karachi ever experienced was 48 degree Celsius on May 9, 1938.