The leaders of the three Sahel nations issued a statement saying it was a "sovereign decision" to leave the Economic Community of West African States "without delay".
Struggling with jihadist violence and poverty, the regimes have had tense ties with ECOWAS since coups took place in Niger last July, Burkina Faso in 2022 and Mali in 2020.
All three were suspended from ECOWAS with Niger and Mali facing heavy sanctions.
They have hardened their positions in recent months and joined forces in an "Alliance of Sahel States".
A French military withdrawal from the Sahel -- the region along the Sahara desert across Africa -- has heightened concerns over the conflicts spreading southward to Gulf of Guinea states Ghana, Togo, Benin and Ivory Coast.
The prime minister appointed by Niger's military regime on Thursday blasted ECOWAS for "bad faith" after the bloc largely shunned a planned meeting in Niamey.
Niger had hoped for an opportunity to talk through differences with fellow states of ECOWAS which has cold-shouldered Niamey, imposing heavy economic and financial sanctions following the military coup that overthrew elected president Mohamed Bazoum.
