The West African bloc ECOWAS imposed sanctions on Niger after army officers toppled President Mohamed Bazoum in a coup last month.
Benin and Nigeria closed their borders, disrupting supplies.
But Burkina Faso, also led by soldiers who came to power in a coup and suspended from ECOWAS, has been quick to show its support for the new regime.
The trucks it sent, mostly carrying food, were guarded by its army before Niger took charge of protecting them, authorities in Niamey said.
Regional customs director Col. Adamou Zaroumeye said "around 300 trucks arrived" in Niamey, adding that the products included maize, salt and household goods.
Seydou Mie Zanaidou, a driver in the convoy, told AFP he had travelled from Kaya in Burkina Faso to Dori in the east, before crossing the Niger border near the town of Tera and pressing on to Niamey.
The United Nations food agency warned last week that sanctions and border closures linked to the political crisis were "greatly affecting the supply of vital foods and medical supplies into Niger."
The U.N. humanitarian agency OCHA highlighted that even before Niger's democratically elected president Mohamed Bazoum was toppled in a coup late last month, the country counted more than three million acutely food-insecure people.
More than seven million others, who are currently considered to be moderately food-insecure, "could see their situation worsen due to the unfolding crisis," it warned, citing a preliminary analysis from the World Food Program.