Multifunctional response to external stimuli which engages various properties, including optical, dielectric, magnetic, or mechanical, can be the source of new generations of highly sensitive sensors and advanced switches. Such responsivity is expected for molecular materials based on metal complexes whose properties are often sensitive to even subtle changes in a particular stimulus. We present a novel hybrid organic–inorganic salt based on earth-abundant divalent manganese ions forming two types of complexes, octahedral cations with methyl-functionalized bis(diphenylphosphino)methane dioxide ligands and tetrahedral anions. These ions crystallize with water molecules leading to the molecular material (1). We show that, due to the simple methyl substituent on the diphosphine-type ligand, 1 reveals a polar crystal structure of the Cc space group as confirmed by the single-crystal X-ray diffraction, second-harmonic generation (SHG) effect, piezoelectric response, and pyroelectricity. Besides these non-centrosymmetricity-related non-linear optical and electrical features, this material combines three other physical properties, i.e., visible room-temperature (RT) photoluminescence (PL) originating from d–d electronic transitions of octahedral Mn(II) complexes, dielectric relaxation in ca. 170–300 K range related to Bjerrum-type orientation defects of water molecules, and slow magnetic relaxation below 3 K related to spin–phonon interactions involving paramagnetic Mn(II) centers. We demonstrate that these three physical effects detected in 1 are sensitive to humidity variation that governs the RT–PL intensity, leads to the ON/OFF switching of dielectric relaxation around RT, and non-trivially modulates the magnetic relaxation at cryogenic temperatures. Thus, we report a unique molecular material revealing broadened multifunctionality and triple physical responsivity to the humidity change exploring luminescent, dielectric, and magnetic properties.