The blast occurred after the Friday prayers at the Imam Ali mosque in al-Qadeeh village in the eastern region of the kingdom.
According to updates on social media, there were about 30 casualties, while several people have been injured in the attack.
At least 150 people were said to be praying at the mosque at the time of the attack.
"We were doing the first part of the prayers when we heard the blast," one worshipper told Reuters.
Security authorities responded to the report of an explosion in a mosque in the Qatif Governorate, the Saudi Press Agency reported.
Security authorities responded to the report of an explosion in a mosque in the Qatif Governorate, the Saudi Press Agency reported.
Television pictures on Lebanon's Hezbollah al-Manar network showed shattered glass and debris next to the tiled pillars inside the mosque. Bodies covered in sheets could be seen laid out on the floor.
There were no immediate details on casualties, but social media images showed blood-covered victims being carried away from the damaged mosque in the village of al-Qadeeh, about 240 miles northeast of the capital, Riyadh.
The Reuters news agency quoted residents saying the attack was carried out by a suicide bomber. The official Saudi News Agency reported an explosion at the mosque, but had no further details.
