The flow and mixing characteristics of three-dimensional confined turbulent round opposing jets in a novel in-line mixer were examined numerically, using air as the working fluid. The computational fluid dynamics (CFD) model was validated with experimental results for a Tee mixer in which a side stream normally impinges on a main stream. Good agreement was obtained between the simulated and experimental results. The effects of key parameters (i.e., jet Reynolds number, dome height, jet injection angle, and side-to-main pipe diameter ratio) on mixing are discussed for this novel opposing jet configuration for passive mixing of fluid streams without the use of internal flow obstructions in the pipe.