Convective turbulence driven by gravitational instability is a fundamental mixing mechanism in geophysical flows, but in situ estimation of its characteristics is obscured by the background flows and the relatively slow temporal scales. We present characteristics of the full Reynolds tensor from a convective surface boundary layer of an ice-covered lake. The results were obtained by using an original method of measuring the full set of turbulent stresses by a combined use of two ADCPs. The strong horizontal shear stress was revealed as a characteristic feature of free convection differing from the “conventional” turbulent boundary flows. The ratio of normal stresses along vertical and horizontal axes remained below 1/4, suggesting strongly anisotropic character of turbulence. The vertical r.m.s. velocity fluctuations obeyed the buoyancy flux scaling with the coefficient of 1/3, which is at lower boundary of the previously reported values.