Previous experimental and analytical studies have revealed that suspended particles can attenuate or enhance turbulence, depending on the particle size in relation to turbulence scales. Incorporating this mechanism, an empirical turbulent eddy viscosity-based closure model is proposed for the mean velocity structure of suspended sediment-laden flow in open channels. The model integrates the sediment particle Stokes number, the ratio of particle-size-to-turbulence microscale, the ratio of particle settling velocity to bed shear velocity, and local sediment concentration. Its good performance is demonstrated in comparison with available laboratory observations. It is characterized that single-phase turbulence closure models can be adapted for sediment-laden flows by implementing sediment particle size effects.