The synthesis of hollow opening polyhedral cages has always been an attractive but challenging goal, especially with regard to inorganic polyhedral cages. Herein, we present a novel, 240-nuclearity giant polymolybdate cage prepared via hydro-thermal synthesis. This cage is composed of 20 tripod-shaped [Mo6O22(SO3)](n-)/[Mo6O21(SO4)](n-) building blocks with three connected vertices and 30 cubane-type [Mo4O16 ](n-) edge building blocks, featuring a rare, nearly regular pentagonal dodecahedron with a large inner cavity (diameter up to 1.8 nm) and 12 opening pentagonal windows. This is the highest nuclearity hollow opening dodecahedral cage reported to date. Importantly, this cage exhibits good stability in solution, as revealed by scanning transmission electron microscopy (STEM), TEM, UV-vis, and Raman spectra. In addition, the bulk sample of this compound exhibits an ultrahigh proton conductivity of 1.03 x 10(-1) S cm(-1) at 80 degrees C and 98% relative humidity, which is the highest among polyoxometalate-based crystalline proton conductors.