"The Iranian regime has been the spearhead of global terrorism since the (Ayatollah Ruhollah) Khomeini revolution" in 1979, King Salman said in a speech to leaders including US President Donald Trump.
"We did not know terrorism and extremism until the Khomeini revolution reared its head," he said.
Saudi Arabia was also determined to "eliminate the ISIS", the king said of the Sunni Muslim jihadist organisation.
The United States and Saudi Arabia on Saturday announced an arms deal worth almost $110 billion, described as the largest in US history.
US Secretary of State Rex Tillerson said the deal was aimed at countering "malign Iranian influence".