The 18 suspects had been “involved in a spying cell for a state”, an interior ministry spokesman told state TV.
“They were gathering information about installations and vital areas in the country and providing intelligence agencies of that state with it,” said ministry spokesman Mansour al-Turki.
He did not name the country thought to be running the spy network.
The arrests were made four days ago and the suspects were being investigated before being handed over to judicial authorities, Mr Turki said.
The arrests were made in “co-ordinated and simultaneous operations in four regions of the kingdom,” including the capital Riyadh and the holy city of Mecca, the ministry was quoted as saying in a separate statement.
Arrests were also made in Eastern Province – where the kingdom’s Shia Muslim minority is concentrated – the statement reportedly said.
Saudi Arabia, which is predominantly Sunni Muslim, has recently repeatedly found itself at odds with Iran, an overwhelmingly Shia Muslim nation.
Tensions deepened last year when Saudi Arabia deployed troops to the Gulf island kingdom of Bahrain to help crush a Shia-led uprising there.