A technique enforcing unidirectional operation of ring lasers is proposed and demonstrated. The approach relies on sum-frequency mixing between a single-pass laser and one of the two counterpropagating intracavity fields of the ring laser. Sum-frequency mixing introduces a parametric loss for the intracavity field copropagating with the single-pass field, effectively generating a loss difference between the copropagating and counterpropagating intracavity fields. This loss mechanism ensures stable unidirectional lasing. The approach is generic and can be implemented at any desired lasing wavelength where lossless second-order nonlinear materials are available. Numerical modeling and experimental demonstration of parametric-induced unidirectional operation of a diode-pumped solid-state 1342 nm cw ring laser are presented.