In order to determine if its magnetic topology is actually two- or three-dimensional (2D or 3D), the mechanism of the magnetic interaction in (5MAP)2CuBr4, a previously thought quasi-2D antiferromagnet, is re-examined using the first-principles bottom-up methodology. Once the magnitude and sign of all unique magnetic interactions present in the room-temperature (5MAP)2CuBr4 crystal are evaluated, it is found that, even at room temperature, the magnetic topology of the crystal corresponds to a 3D antiferromagnet. Such 3D nature cannot be determined by examination of the magnetic susceptibility curve, χ(T), because it is found that the χ(T) curve computed using this 3D magnetic topology is very similar to that obtained using a 2D model where all interplane interactions have been deleted. However, its 3D magnetic dimensionality can be confirmed by examination of the shape of the magnetization curve, M(H); the computed curve is similar to the experimental one for the 3D case.