The title compound, potassium silver tetra-niobium nona-oxide arsenate, K(0.8)Ag(0.2)Nb(4)O(9)AsO(4), was prepared by a solid-state reaction at 1183 K. The structure consists of infinite (Nb(2)AsO(14))(n) chains parallel to the b axis and cross-linked by corner sharing via pairs of edge-sharing octa-hedra. Each pair links together four infinite chains to form a three-dimensional framework. The K(+) and Ag(+) ions partially occupy several independent close positions in the inter-connected cavities delimited by the framework. K(0.8)Ag(0.2)Nb(4)O(9)AsO(4) is likely to exhibit fast alkali-ion mobility and ion-exchange properties. The Wyckoff symbols of special positions are as follows: one Nb 8e, one Nb 8g, As 4c, two K 8f, one Ag 8f, one Ag 4c, one O 8g, one O 4c.