The assailants "arrived on motorbikes" in the village of Darey-Daye in the Tillaberi region on Monday afternoon as people were working in the fields, a local official said.
"The toll is very high -- there were 37 dead, including four women and 13 children," the source said.
A local journalist confirmed the toll and described the attack as "very bloody".
"They found people in the fields and shot at anything that moved," he said.
The deaths bring the unofficial death toll from terrorist attacks in western Niger to more than 450 since the start of the year. It is also the fifth attack in this area of Tillaberi in as many months, claiming 151 lives.
Rated the world's poorest country by the UN's Human Development Index, Niger lies in the heart of the arid Sahel region of West Africa, which is battling a nine-year-old terrorist insurgency.
The bloodshed began in northern Mali in 2012 and then spread to the centre of the country before hitting neighbouring Niger and Burkina Faso.
Tillaberi has borne the brunt of the crisis.
Darey-Daye, located 40 kilometres (25 miles) east of the town of Banibangou, was already reeling from a bloody assault on March 15.
Suspected terrorists killed 66 people in attacks on the village and on vehicles of shoppers returning from the weekly market in Banibangou.
According to a toll issued last Wednesday by Human Rights Watch (HRW), more than 420 civilians have been killed in terrorist attacks in Tillaberi and the neighbouring region of Tahoua this year.
Tens of thousands of people have fled their homes.
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