He was killed by unidentified attackers in a house where he was staying, his son, Maulana Hamidul Haq said.
He was shifted to the Safari Hospital in Rawalpindi for medico-legal formalities.
Yousaf Shah, Haq's spokesman, said the attacker's identity and their motive were not known. On his return, he saw that Maulana Sami was lying in his bed in a pool of bed.
A divisive figure, Haq was known as the "Father of the Taliban". The army chief also extended his condolences to the bereaved family.
Speaking to the television channel, Hamidul Haq said Maulana Samiul was unable to participate in the protests against Asia Bibi's acquittal by the Supreme Court in a blasphemy case.
PTI chairman and Prime Minister Imran Khan has condemned the assassination of Sami, stating that the country had been deprived of an important religious leader.
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In a statement from Beijing, where he is now on the visit to China, the prime minister directed to investigate the incident and bring the perpetrators to justice. The prime minister said Maulana Sami-ul-Haq was a great scholar of Islam.
A security guard of Sami ul Haq also has sustained serious injury, the report said.
On the other hand, Pakistan Muslim League-Nawaz (PML-N) supremo Nawaz Sharif and party president Shehbaz Sharif also condemned Haq's assassination.
Maulana Sami-ul-Haq was director of madrassa Darul Uloom Haqqania in Nowshera distrcit of Khyber Pakhtunkhwa Province of Pakistan which was a breeding ground for many top Taliban leaders including Mullah Omar. There he studied fiqh, usul al-fiqh, Arabic literature, logic, Arabic grammar (sarf and nahw), tafsir, and Hadith.
The Senate website describes him as: "A qualified religious scholar, an educationist, a research scholar and a lieutenant of his father, late Maulana Abdul Haq".
Samiul Haq was now the chancellor of Darul Uloom Haqqania, a Deobandi Islamic seminary which is the alma mater of many prominent Taliban members.