Mali, Burkina Faso, and Niger have introduced a new 0.5% tax on imports from Nigeria and other Economic Community of West African States (ECOWAS) members.
The three countries, which exited ECOWAS last year, announced in a joint statement that the levy would take immediate effect.
It applies to all imports from outside their alliance but excludes humanitarian aid.
The funds raised will support the operations of their emerging economic bloc, though specific details on spending were not disclosed.
This tariff effectively ends unrestricted trade among West African nations under ECOWAS, further emphasizing the growing division between the Sahel states and regional powerhouses like Nigeria and Ghana.
Currently governed by military regimes following coups in 2023, Mali, Burkina Faso, and Niger initially formed the Alliance of Sahel States for security cooperation.
The group has since evolved into a budding economic union with ambitions of military and financial integration, including plans to introduce biometric passports.
The three nations left ECOWAS, accusing it of failing to adequately assist them in tackling Islamist insurgencies.
In response, the regional bloc imposed economic and political sanctions to pressure them into restoring democratic governance, but the measures have had little impact.