A new semi-inverse design method for turbomachinery blading is proposed in this paper. Built on a time-marching Reynolds-Averaged Navier-Stokes solver, the proposed design method takes pressure loading, blade tangential thickness, blade stacking line, and flow path contour as prescribed quantities and computes the corresponding three-dimensional blade camber surface. In order to have the option of imposing geometrical constraints on the designed blade shapes, a new algorithm is developed to solve the camber surface at specified spanwise grid-lines, after which the blade geometry is constructed through ruling (e.g. straight-line element) at the remaining spanwise stations. The new semi-inverse algorithm involves re-formulating the boundary condition on the blade surfaces as a hybrid inverse/analysis boundary condition while preserving the fully three-dimensional nature of the flow field. The new design method can be interpreted as a fully three-dimensional viscous semi-inverse method. The ruled camber surface design procedure ensures blade surface smoothness and some control of mechanical integrity, and results in cost reduction for the manufacturing process. The proposed fully three-dimensional semi-inverse method is demonstrated through design modifications of generic industrial mixed-flow and radial impellers which are typically used for gas process applications.