Wasilu Umar admitted killing her husband, who was more than twice her age, by concealing rat poison in his food, the police in Kano said.
Three other people also died and 10 were taken to hospital apparently after eating the same food.
Child marriage is common in Nigeria, especially in the mainly Muslim north.
The girl's father had forced her to marry the 35-year-old man, police said.
The marriage took place last week, police superintendent Musa Magaji Majia told the Associated Press news agency.
The BBC's Will Ross in Nigeria says Islamic police are trying to stop parents from forcing children into marriages against their will and the father could be charged.
The teenager is likely to be tried in a juvenile court.
A 14-year-old girl in the northern Nigerian state of Kano has confessed to killing the man she was forced to marry, police say.