A tantalum oven that produces quiescent isothermal cesium plasmas at precisely known temperatures and pressures is described. The apparatus consists of a closed, cesium vapor filled tube heated by radiation within a resistance heated vacuum furnace. Plasma temperatures up to 2300°K and cesium vapor pressures up to 10 Torr are obtainable. Temperatures are measured by calibrated unsheathed W/W‐26%Re thermocouples. Cesium vapor pressures are known through careful control of the cesium condensation conditions. Designed especially for spectroscopic studies, the oven features two sapphire windows mounted on a line‐of‐sight axis through the plasma. Details of construction and operation are described. Performance data showing temperature profiles to 1% over a 10 cm length are given.