An aluminogermanate, Na₂Al₂Ge₃O₁₀·2H₂O (NaAlGe-NAT), possessing the natrolite topology has been synthesized under hydrothermal conditions at 423 K from sodium- and tetramethylammonium-containing gels. Room temperature single crystal X-ray diffraction data indicate a tetragonal unit cell with a = 13.314(2) and c = 6.819(2) Å, and space group . NaAlGe-NAT displays complete disorder of aluminium and germanium in the framework, with Ge/Al = 1.5. The substitution of germanium for silicon in the framework leads to a larger unit cell volume for NaAlGe-NAT compared with Na₈Al₈Si₁₂O₄₀·8H₂O (NaAlSi-NAT) and Na₈Ga₈Si₁₂O₄₀·8H₂O (NaGaSi-NAT) framework structures. The mean rotation angle Ψ of the chains (composed of 4=1 secondary building units) relative to the a and b cell axes is 20.4°, compared with 21.7 and 22.9° calculated for NaGaSi-NAT and NaAlSi-NAT, respectively. Elliptical channels along the c-axis host well-ordered water molecules and sodium cations in fully occupied sites.