The stability and bulk mixing characteristics of an axisymmetric turbulent buoyant jet discharging vertically into a stagnant waterbody of large horizontal extent is studied theoretically and experimentally. A stable discharge configuration is defined as one in which a buoyant surface layer is formed which spreads radially from the source and does not communicate with the initial buoyant jet region. On the other hand, the discharge configuration is unstable when recirculation cells exist around the jet efflux. A semi-empirical theory shows the discharge stability is only dependent on the dynamic interaction of three near-field regions—the buoyant jet region, surface impingement region, and radial internal hydraulic jump region. A series of laboratory experiments were performed with a half-jet, using heat as source of buoyancy, and inserting a plane of symmetry in a large model basin. The experimental results are in good agreement with the theoretical predictions, both as regards the stability criterion and the near field mixing characteristics.