Interior Ministry spokesman Mansour al-Turki said on state television the suspects were “involved with a spy network working for a foreign country”.
Turki later told Reuters he could not say who the 18 were suspected of spying for, pending further investigation.
Sunni Saudi Arabia and Shi’ite Iran have tussled for influence across the Middle East, including in Lebanon, and Riyadh has accused Tehran of stoking unrest in neighbouring Bahrain, a charge it has denied.
The spying arrests, the first in recent memory, follow a string of accusations by Riyadh that an unnamed foreign country, widely understood to mean Iran, had instigated protests inside Saudi Arabia.
The arrests were made four days ago and the suspects were being investigated before being handed over to judicial authorities, Turki said on television.
“They were gathering information about installations and vital areas in the country and providing intelligence agencies of that state with it,” he added.