A new type of polyoxomolybdate containing mixed-valence arsenic, [AgAs^III₂(As^IIIAs^VMo₄O₁₈(OH)₂)₃]¹¹⁻ (1), has been synthesized. Single-crystal X-ray analysis was carried out on (NH₄)₅(C₂H₉N₂)_4.5H_1.5[AgAs^III₂(As^IIIAs^VMo₄O₁₈(OH)₂)₃]·14H₂O (1a) and (NH₄)₁₁[AgAs^III₂(As^IIIAs^VMo₄O₁₈(OH)₂)₃]·16H₂O (1b). The polyanion is composed of three unprecedented [As^IIIAs^VMo₄O₁₈(OH)₂]⁶⁻ subunits linked together by two AsO₃ trigonal-pyramidal structures from both sides via sharing six bridging oxygen atoms, and a silver cation is enclosed in the center of the polyanion and coordinated by three As atoms with the As–Ag–As bond angle of 120°, to lead to a fan-like structure with an approximative C_3h symmetry. The polyanion was also characterized by IR, X-ray photoelectron spectroscopy and thermogravimetry-differential scanning calorimetry. The stability of [AgAs^III₂(As^IIIAs^VMo₄O₁₈(OH)₂)₃]¹¹⁻ in an aqueous solution was investigated by electronic absorbance spectroscopy and electrospray ionization mass spectrometry, and the possible disaggregation mechanism of the polyanion is proposed.