“One person killed in Najran mosque bombing, 12 injured and the suspect is injured,” Al-Ekhbariya news channel reported.
The interior ministry confirmed a blast had occurred but had no immediate details.
“A person went to the mosque and did an explosion,” a spokesman told AFP.
The television report did not specify whether the mosque belonged to the kingdom’s Sunni majority or the Shia minority.
But roughly half of Najran’s population belongs to the Ismaili Shia community.
Most of Saudi Arabia’s Shias live in Eastern Province, where they have been targeted four times in the past year by bombings and shootings linked to the Islamic State group of Sunni extremists, who consider them heretics.
The latest attack in the east occurred on October 16 when a gunman fired on Shias commemorating Ashura in the Qatif area.
Ashura is one of the holiest occasions for the Shia faith.
The gunman was shot dead by police.
In June, four Shias died preventing a suicide bomber from entering the hall of Al-Anoud mosque in Dammam city.
Days earlier, 21 people were killed in another Shia mosque bombing in Eastern Province.
Groups claiming affiliation with IS said they carried out the earlier bombings and the Ashura shooting.
During Ashura last year, gunmen killed seven Shia worshippers, including children, in the eastern town of Al-Dalwa.