They said the party had been close to al-Sistani for some time, but a two-day conference on Baghdad that ended on Friday had formalised relations with the influential cleric.
Rida Jawad al-Takki, a senior group member, read out the party's decisions to reporters.
"We cherish the great role played by the religious establishment headed by Grand Ayatollah Sayed Ali al-Sistani ... in preserving the unity of Iraq and the blood of Iraqis and in helping them building a political system based on the constitution and law," he said.
He also said the party pledged to follow the guidance of the Shia establishment.
Spiritual leadership
Al-Sistani, a reclusive figure who lives in the Najaf, is the spiritual leader of Iraq's majority Shia. He rarely makes public statements but his utterances are closely monitored by his followers.
Officials said the party, which was formed in Iran in the 1980s to oppose Saddam, had previously taken its guidance from the religious establishment of Welayat al Faqih, led by the Supreme Leader Ayatollah Ali Khamenei in Iran.
Islamic experts say the authority of the Faqih, who "surpasses all others in knowledge" of Islamic law and justice, is not limited to his home country, but extends to all Shia Muslims who pledge obedience and believe in the Faqih.
The Faqih has the final word on matters related to Islam from political, social and religious issues.
Al-Hakim
Abdul Aziz al-Hakim, leader of SIIC, is a powerful religious leader who has good relations with the US.
A key player in post-Saddam Iraqi politics, SIIC holds around a quarter of the seats in parliament occupied by the ruling Shia Alliance of Nuri al-Maliki, the prime minister. Iraq and Iran fought a bitter war for eight years in the 1980s.
Relations have improved since the fall of Saddam, although Iraqi leaders have to walk a delicate line between the United States and Iran, which are at loggerheads over Tehran's nuclear programme and the violence in Iraq.