The incident took place during noon prayers at the Habib Ibn al-Mudhaher mosque in the predominantly Shiite neighborhood of al-Qahira in the northeastern section of the Iraqi capital, CNN reported.
Police said the two bombers used pistols equipped with silencers to kill guards before entering the mosque and blowing themselves up.
The bombing follows a string of attacks across central Iraq Monday that killed at least a dozen people and wounded more than 50 others.
Monday was a special voting day for security forces, police and others ahead of provincial elections.
Tensions between Sunnis and Shiites have risen in recent months.