According to state media, King Salman bin Abdulaziz has been admitted to hospital for treatment of inflammation of the gall bladder.
He is 84 and has ruled Saudi Arabia since 2015. His son Mohammed bin Salman is next in line.
Iraqi Prime Minister Mustafa al-Kadhimi postponed his scheduled visit to Riyadh following the king’s hospitalisation, the Saudi foreign minister said.
King Salman, the custodian of Islam’s holiest sites, spent more than two and a half years as the Saudi crown prince and deputy prime minister from June 2012 before becoming king. He also served as governor of the Riyadh region for more than 50 years.
The crown prince, Mohammed bin Salman has won praise for easing social restrictions giving more rights to women and pledging to diversify the economy.
But he has also been criticized for Saudi Arabia’s involvement in Yemen’s long-running war and attempts to silence dissident and consolidate power by marginalising rivals, including a purge of top royals and businessmen on corruption charges.
In 2018, he was in the spotlight because of the murder of Saudi journalist Jamal Khashoggi in the kingdom’s Istanbul consulate, which the CIA has reportedly said took place on the crown prince’s orders.
The crown prince has denied ordering Khashoggi’s killing but said he ultimately bears “full responsibility” as the kingdom’s de facto leader.