Alluaudia montagnacii is a rare species of flowering plant in the family Didiereaceae.

The tall columnar central trunk sprouts strong vertical branches.

Alluaudia montagnacii is endemic to the Mahafaly Plateau of southwestern Madagascar.

[1] It grows in small arid and subarid areas on a narrow belt of the coast.

It is part of the Madagascar spiny forests, where it forms a specific plant community with species such as Cedrelopsis grevei and Ficus marmorata.