The title compound, silver disodium trimolybdenum(VI) nonaoxide arsenate, AgNa(2)Mo(3)O(9)AsO(4), was prepared by a solid-state reaction at 808 K. The structure consists of an infinite (Mo(3)AsO(13))(n) ribbon, parallel to the c axis, composed of AsO(4) tetra-hedra and MoO(6) octa-hedra sharing edges and corners. The Na and Ag ions partially occupy several independent close positions, with various occupancies, in the inter-ribbon space delimited by the one-dimensional framework. The composition was refined to Ag(1.06(1))Na(1.94(1))Mo(3)O(9)AsO(4).