We report the synthesis and structural characterization of the ABX3 perovskite frameworks [C(NH2)(3)]Mn-1-x(2+)(Fe-2x/3(3+),square(x/3))(HCOO)(3) (square = B-site vacancy). For large x, the vacancies order, lowering the crystal symmetry. This system establishes B-site vacancies as a new type of defect in formate perovskites, with important chemical, structural, and functional implications. Monte Carlo simulations driven by nearest-neighbor vacancy repulsions show checkerboard vacancy order to emerge for x > 0.6, in accord with experiment.