Sheikh Abdul Latif Abdul Aziz al-Sheikh said anyone using social media sites - and especially Twitter - "has lost this world and his afterlife".
Twitter was the platform for those who did not have any platform, he said.
His remarks reflect Riyadh's concern that Saudis use Twitter to discuss sensitive political and other issues.
The conservative kingdom is believed to have seen the world's fastest increase in the uptake of Twitter, says the BBC's Sebastian Usher.
The sheikh's comments echo those of the imam of the Grand Mosque in Mecca in April who used his sermon - seen by millions on TV - to warn that Twitter was a threat to national unity, our correspondent says.
Earlier, Saudi Arabia's grand mufti, the kingdom's most senior Muslim cleric, had dismissed Twitter users as "fools".