The attack on Wednesday came as at least four other Iraqis were killed and another four wounded near the southern city of Kut.
The Kut deaths occurred when a minibus carrying civilians was allegedly caught in crossfire between US forces and resistance fighters.
In Mosul, policeman Muzahim Abd Allah al-Shimmari said a mortar round struck the Bab al-Tub market at around 2:00pm (1000 GMT).
Five bodies and 20 of the injured were admitted to hospital after the attack, an official said.
Responsibility for the Mosul bombing has not been claimed.
Students shot dead
In Kut, Iraqi police said five women students were killed and another five wounded when a US convoy was attacked at around 8:30am outside the southern city, AFP reported
"US troops returned fire just as a minibus carrying the students passed by. Five of them were killed and five wounded, three of whom are in a critical condition," said Lieutenant Husayn Abbas Rudhan.
But a statement from the US-led occupation forces in Iraq said four passengers in the bus were killed and four wounded when the bus was caught in crossfire.
Resistance fighters loyal to Shia leader Muqtada al-Sadr have previously clashed with security forces in and around Kut, but it was not immediately clear if the al-Mahdi Army was involved in the shootout.