The bomber detonated explosives soon after entering the Abu Bakr al-Sadiq Mosque as the imam gave the Friday sermon in the town of Al-Wajihiyah, a police colonel said.
A doctor confirmed the toll from the blast which also wounded 40 people.
"I was sitting near the main entrance of the mosque when a huge explosion happened," sending dirt and objects flying, said Sinan Ghalib, who woke up in a hospital in the nearby city of Baquba with a serious wound in his left leg.
"I was sitting near the imam and the mosque was full of dozens of people when a big explosion happened, and the place went completely dark," Mundhir said, also in hospital.
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"I found myself on the ground at the hospital later, with many other wounded" who were screaming and crying, he said.
Areas near Baquba have been hit by several attacks over the past few days, including a bombing on Tuesday that targeted worshippers leaving a Sunni mosque in Muqdadiyah, northeast of the city, killing four people and wounding 15.
A bombing targeting a Shiite religious hall in north Baghdad earlier this month killed at least 15 people.
The latest unrest brings the number of people killed in violence this month to 457, and upwards of 2,700 since the beginning of the year.
The death toll for the first 19 days of July alone has already surpassed the total for the entire month of June.
Most senior Iraqi politicians and religious leaders have remained silent about the current wave of violence.