The committee that issues religious rulings, headed by Grand Mufti Shaikh Abd al-Aziz al-Shaikh, "urges citizens and (foreign) residents to inform on anyone planning or preparing to carry out an act of sabotage", said the fatwa, carried by the state SPA news agency.
The aim is to "protect the people and the country from the destructive effects of such actions and to shield the planners themselves from the consequences of their actions", it said on Friday.
The edict also called on Islamists to "fear God Almighty and come back to their senses".
The committee said it had issued its edict in response to inquiries about "the appalling events of the past few weeks", which have seen an escalation in violence blamed on sympathisers of the al-Qaida group.
Authorities have promised hefty financial rewards for those who help catch most-wanted individuals or thwart attacks.
More than 85 people have been killed and hundreds injured in the wave of violence which began in May 2003. In one of the bloodiest episodes, 22 people were killed when dissidents went on a shooting rampage and seized hostages in the oil city of al-Khobar last weekend.
Friday's edict reinforces repeated calls by Saudi officials on the population to inform on terror suspects, coupled with warnings that those who turn a blind eye to violent activities will be seen as accomplices.