The attack took place during prayers in the Umm al-Qura mosque in west Baghdad, the city's largest Sunni mosque.
Officials said parliamentarian Khalid al-Fahdawi was among the dead in the strike, the Associated Press reported.
The attack comes towards the end of the Muslim holy month of Ramadan.
The mosque is the main headquarters of the Sunni Endowment, which is responsible for maintaining Sunni Muslim religious sites across Baghdad.
"A suicide bomber entered the main area of the mosque and blew himself up," said Qutaiba al-Falahi, a spokesman for the group, according to Reuters news agency.
No group immediately claimed responsibility for Sunday's bombing.
There has been a recent flurry of violence across Iraq, although it is much reduced since a peak in 2006-2007.
On Friday at least 13 people were killed in a series of attacks across the country - in Basra, Falluja and Baghdad.
A suicide bomber has killed at least 28 people and wounded at least 30 more after blowing himself up inside a Sunni mosque in the Iraqi capital Baghdad, local officials say.