Single crystals of Na2Ba9Si20O50 were obtained from solid state reactions performed along the join Na2Si2O5-BaSi2O5. The crystal structure has been determined from a data set collected at ambient temperatures and subsequently refined to a residual of R(|F|)=0.0328 for 2211 independent reflections. The compound belongs to the group of phyllosilicates and adopts the monoclinic space group C2/m with the following lattice parameters: a=39.111(3)angstrom, b=7.6566(6)angstrom, c=8.2055(6)angstrom, beta=97.319(6)degrees, V=2437.2(3)angstrom 3, Z=2. Furthermore, weak one-dimensional diffuse streaks running parallel to a* as well as a very small number of low intensity reflections at b*/3, indicating the presence of a superstructure, were observed. Basic buiding units are silicate layers parallel to (40-1) which can be obtained from the condensation of single chains with a periodicity of four running along [010]. The sheets can be partitioned into two kinds of consecutive strips containing (i) a sequence of four- and eight-membered rings and (ii) a four-ring wide zig-zag shaped unit consisting of exclusively six-membered rings. The sodium and barium cationsdistributed among six crystallographically independent positionsare sandwiched between subsequent layers and are linked to seven to nine nearest oxygen neighbors. The structure of Na2Ba9Si20O50 is closely related to that of K2Ba5Si12O30 and K2Ba7Si16O40, respectively. There are strong arguments that the previously claimed phase Na4Ba8Si20O50 is actually misinterpreted Na2Ba9Si20O50 and that the composition of the intermediate phase along the join Na2Si2O5BaSi2O5 is slightly different from that described in the literature.