The 14-year-old girl in the northern Nigerian state of Kano has confessed to killing the man, who was more than twice her age.
Wasilu Umar said she fed her husband poison after he invited a dozen friends to celebrate their marriage at his Ungwar Yansoro village.
"The suspect confessed to committing the crime and said she did it because she was forced to marry a man she did not love," police superintendent Musa Magaji Majia told the Associated Press.
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 impoverished north.
Early and forced marriage is classified as modern-day slavery by the UN labour organisation, and Nigeria's Child Rights Act prohibits marriage before 18. But that federal law competes with Islamic Shariah law that holds in most northern states.
The girl's father had forced her to marry the 35-year-old man, police said.
The teenager is likely to be tried in a juvenile court, the BBC reported. The teenager is co-operating with police and likely will be charged with culpable homicide, according to Majia.