The Public Health Ministry said that at least 45 others were wounded.
Faredoon Obiadi, head of the criminal investigation department for the Kabul police, said the attacker was on foot and detonated his suicide vest among the crowds inside the Baqir-ul Ulom mosque in western Kabul.
The attack took place on the first floor of the two-story building where Shiite worshippers had gathered to commemorate the death of Imam Hussein, the Prophet Muhammad's grandson and an iconic Shiite martyr, in Karbala, Iraq in 680 AD.
No group has yet claimed responsibility but militant Sunni fundamentalists like the Taliban and the Islamic State group view Shiites as apostates and frequently attack Shiite mosques and public gatherings.
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Today's bombing struck a ceremony commemorating 40 days since the anniversary of Hussein's death.
In early October, at a gathering commemorating the actual death anniversary, militants attacked another Shiite shrine in Kabul, killing 14.
Shiites in Afghanistan make up an estimated 15 per cent of the population of around 30 million. Their public celebrations and commemorations were largely banned during the five years when the Taliban controlled the country.