A modular wall system includes a plurality of extruded wall panels each having opposing first and second surfaces and opposing first and second joiner side edges. Each joiner side edge has an edge surface adjacent the first surface, another edge surface adjacent the second surface and a tongue or groove disposed therebetween. The edge surfaces have an angle less than 90 degrees with respect to the first or second surface, respectively. The first joiner side edge is insertable into the second joiner side edge of another wall panel, with a flush fit, for interconnecting the wall panels. Each panel also has opposing top and bottom edges and a plurality of ribs connecting and perpendicular to the first surface and the second surface.