The 79-year-old had been facing severe health issues ever since assuming charge as the 31st governor of the province on November 11 last year.
Governor Saeed-uz-Zaman Siddiqui had also served as a High Court Judge in Sindh.
Sindh Assembly Speaker Agha Siraj Durrani will serve as acting governor of Sindh until the appointment of new governor. Siddiqui, as retired chief justice of Pakistan, was sworn in as the 31st Governor of Sindh in November 2016, just days after the federal government chose to end the 14-year tenure of former Sindh governor Ishratul Ibad.
Siddiqui was one of the 100 or so judges who refused to take oath under the PCO, declaring it a deviation from the oath they took under the Constitution and a negation of the independence of the judiciary.
Pakistan Muslim League-Nawaz (PML-N) during the 2008 presidential election of Pakistan named Justice (retd) Siddiqui as a party candidate to replace Pervez Musharraf as President of Pakistan.
Prime Minister (PM) Nawaz Sharif and President Mamnoon Hussain have expressed sorrow over the demise while leaders of various political parties have acknowledged his contribution to national politics in his career. His last reported official activity was on 19 December, 2016 when he met at the Governor House the new corps commander of Karachi Lt General Shahid Baig Mirza.
He said Siddiqui had never compromised over principles and always confronted non-democratic characters.
In a statement, he prayed eternal peace for the departed soul and commiserated with the bereaved family.
Chief Minister Sindh Syed Murad Ali expressed deep grief and sorrow over the sad demise of Governor Sindh Justice rtd Saeeduzaman Siddiqui and has announced a day long mourning on Thursday. He added that the family of the governor must be consulted in each and every decision.
Justice Siddiqui was awarded honorary membership of the judicial fraternity of Australia and Canada in January 2000.
Punjab Governor Malik Muhammad Rafique Rajwana on Wednesday expressed deep grief and sorrow over the sad demise of Governor Sindh Justice (retd) Saeed-uz-Zaman Siddiqui. Provincial Minister prayed that may almighty Allah rest the departed soul in eternal peace and grant courage and fortitude to bereaved family members to bear this irreparable loss.
Pakistan People's Party (PPP) Chairman Bilawal Bhutto Zardari also expressed grief over death of the governor.
Born in Lucknow of British India, late Saeed uz-Zaman Siddiqui educated himself at University of Dhaka and University of Karachi.