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The elastic behavior and the phase stability of natural pollucite, (Cs, Na)₁₆Al₁₆Si₃₂O₉₆·nH₂O, were investigated at hydrostatic pressure by in situ single-crystal X-ray diffraction with a diamond-anvil cell. Pollucite experiences a P-induced phase transition, not previously reported in the literature, at P = 0.66 ± 0.12 GPa from cubic (Ia3̅d) to triclinic symmetry (P1̅). The phase transition is completely reversible and without any appreciable hysteresis effect. No further phase transition has been observed up to 9 GPa. Fitting the pressure-volume data of the low-pressure cubic polymorph with a second-order Birch-Murnaghan Equation-of-State (BM-EoS), we obtain V₀ = 2558.3(4) ų, K_T0 = 41(2) GPa, and K_T′ = 4 (fixed). For the high-pressure triclinic polymorph, a third-order BM-EoS fit gives V₀ = 2577.5(40) ų, K_T0 = 25.1(9) GPa, and K_T′ = 6.5(4). The axial bulk moduli of the high-pressure triclinic …