According to his son, Hamidul, Haq was stabbed to death in his bedroom at home in Rawalpindi.
Saeed-ur-Rehman Sarwar, a leader of Haq's JUI-S political party, also confirmed the killing. He said Maulana Sami-ul-Haq was a great scholar of Islam.
Punjab Chief Minister Usman Buzdar has also denounced the assassination and sought a report from the Inspector General Punjab Police.
Media reports said that a security guard of the deceased cleric was critically wounded.
Haq was known as the "Father of the Taliban" because he trained some of the leaders of the Islamist movement in Afghanistan.
No individual or group has so far claimed responsibility behind the assassination of Maulana Samiul Haq.
Haq's sprawling Haqqani seminary taught many of Afghanistan's Taliban and thousands of other students a strict interpretation of Islam.
He began his education in 1366 AH (1946 or 1947 CE) at Darul Uloom Haqqania, which was founded by his father.
He was well versed in Arabic but also used Urdu, the national language of Pakistan, and the regional language of Pashto.
He has twice been a member of the Senate of Pakistan from 1985 to 1991 and again from 1991 to 1997.
Human Rights Minister Shireen Mazari said the brutal murder of Maulana Samiul Haq was "utterly condemnable".
He was a religious scholar and former Senator.
DW interviewed Haq in 2015, during a visit to Haqqania.
The news of his death comes at a sensitive time, with religio-political parties out on the streets protesting the Supreme Court's acquittal of Asia Bibi, a Christian woman, in a blasphemy case.