A Missing Member of Well-Known Ternary Chalcometallates, a Sodium Selenogallate, NaGaSe2, Has Been Synthesized by Employing a Polyselenide Flux and Stoichiometric Reaction. Crystal Structure Analysis using X-Ray Diffraction Techniques Reveals that It Contains Supertetrahedral Adamantane-Type Ga4Se10 Secondary Building Units. These Ga4Se10 Secondary Building Units Are Further Connected Via Corners to Form Two-Dimensional (2D) [GaSe2]∞- Layers Stacked Along the C-Axis of the Unit Cell, and the Na Ions Reside in the Interlayer Space. the Compound Has an Unusual Ability to Absorb Water Molecules from the Atmosphere or a Nonanhydrous Solvent to Form Distinct Hydrated Phases, NaGaSe2·xH2O (Where X Can Be 1 and 2), with an Expanded Interlayer Space, as Verified by X-Ray Diffraction (XRD), Thermogravimetric-Differential Scanning Calorimetry (TG-DSC), Desorption, and Fourier Transform Infrared Spectroscopy (FT-IR) Studies. the in Situ Thermodiffractogram Indicates the Emergence of an Anhydrous Phase Before 300 °C with the Decrease of Interlayer Spacings and Reverting to the Hydrated Phase within a Minute of Re-Exposure to the Environment, Supporting the Reversibility of Such a Process. Structural Transformation Induced through Water Absorption Results in an Increase of Na Ionic Conductivity by 2 Orders of Magnitude Compared to that of the Pristine Anhydrous Phase, as Verified by Impedance Spectroscopy. Na Ions from NaGaSe2 Can Be Exchanged in the Solid-State Route with Other Alkali and Alkaline Earth Metals in a Topotactic or Nontopotactic Way, Leading to 2D Isostructural and Three-Dimensional Networks, Respectively. Optical Band Gap Measurements Show a Band Gap of ∼3 EV for the Hydrated Phase, NaGaSe2·xH2O, Which is in Good Agreement with the Calculated Band Gap using a Density Functional Theory (DFT)-Based Method. Sorption Studies Further Confirm the Selective Absorption of Water over MeOH, EtOH, and CH3CN with a Maximum Water Uptake of 6 Molecules/formula Unit at a Relative Pressure, P/P0, of 0.9.