This paper presents a study on the verification and validation (V&V) of numerical solutions for round buoyant jets in counterflow. The unsteady flow was simulated using an unsteady Reynolds-averaged Navier–Stokes (URANS) solver with a two-phase mixture model. This work aimed to quantitatively investigate the reliability and applicability of various uncertainty estimators in the simulation of a buoyant jet in counterflow. Analysis of the discretization uncertainty estimation results revealed that the factor of safety (FS) and the modified FS (FS1) methods were the appropriate evaluation estimators in the simulation of a buoyant jet in counterflow. Validation by comparison with the experimental data indicated that the area without achieving the validation at the validation level was strongly related to the shear layer between the jet flow and the ambient fluid. Moreover, the predicted concentration contours, coherent structures, and centerline concentration were strongly affected by the grid resolution.