A police officer says the attacker blew up his explosives-laden vest as people were leaving the al-Khairat mosque in Baghdad's Sinak commercial area following noon prayers on Monday.
He says at least 25 people were wounded in the explosion.
Monday's attack comes a day after a suicide bombing targeted a Shi'ite mosque in the capital's western Harithiya neighbourhood, killing 28 worshippers.
There was no immediate claim for Sunday's blast, but suicide bombings are almost exclusively carried out by Sunni jihadists in Iraq, including members of the Islamic State (IS) group, which has seized large areas of Iraq since June.
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The militants have been kept from reaching Baghdad in force and would face major challenges if they tried to occupy part of the city, but they are able to strike the capital with deadly bombings.
Members of Iraq's Shi'ite majority, whom Sunni extremists consider to be heretics, have suffered the brunt of the blasts.
