Road accidents are common in Africa's most populous country and largest economy, typically caused by reckless driving, speeding, over-loading or lack of maintenance.
Because of the rainy season, there have also been thunderstorms and flooding across the country in recent weeks, increasing the risk of accidents.
"At about 6am (0500GMT), we received a distress call from Radabi village that an accident occurred on the Gwaram-Basirka road," police spokesman Lawan Shiisu said in a statement.
He said a hummer bus and a canter lorry carrying the passengers "fell into a broken culvert full of water".
Victims were evacuated to a nearby hospital where 21 were confirmed dead, Shiisu added.
One of the passengers, a 26-year-old male, survived the accident, police said.
Sixteen people were killed in August last year when a truck lost control in heavy rain and rammed into four vehicles carrying supporters of Zamfara state governor Bello Matawalle.
