We report the synthesis, crystal structure, and thermal, dielectric, phonon, and magnetic properties of [NH2-CH+-NH2][Mn(HCOO)(3)] (FMDMn). The anionic framework of [(Mn(HCOO)(3)(-)] is counterbalanced by formamidinium (FMD+) cations located in the cavities of the framework. These cations form extensive N-H center dot center dot center dot O hydrogen bonding with the framework. The divalent manganese ions have octahedral geometry and are bridged by the formate in an anti anti mode of coordination. We have found that FMDMn undergoes a structural phase transition around 335 K. According to the X-ray diffraction, the compound shows R (3) over barc symmetry at 355 K and C2/c symmetry at 295 and 110 K. The FMD+ cations are dynamically disordered in the high-temperature phase, and the disorder leads to very large bandwidths of Raman and IR bands corresponding to vibrations of the NH2 groups. Temperature-dependent studies show that the phase transition in FMDMn is associated with ordering of the FMD+ cations. Detailed analysis shows, however, that these cations still exhibit some reorientational motions down to about 200 K. The ordering of the FMD+ cations is associated with significant distortion of the anionic framework On the basis of the magnetic data, FMDMn is a weak ferromagnet with the critical temperature T-c = 8.0 K.