The Al-Ekhbariya news channel quoted an interior ministry spokesman as giving the new toll, up from two dead and seven wounded earlier.
Residents said the mosque belonged to the minority Shiite community. In a written statement, the ministry said security personnel prevented two bombers from getting inside the Al-Rida mosque in the Mahasen neighbourhood of Al-Ahsa city during the main weekly prayers.
"When security approached to intercept them, one of them responded by blowing himself up at the mosque entrance while an exchange of fire took place with another," it said.
"He was injured and arrested. A suicide belt was found in his possession."
Since deadly attacks claimed by the Islamic State group last year against Shiite mosques in the kingdom's east, security has been increased, including with the presence of community guards who inspect visitors to houses of worship.