A uniform asymptotic high-frequency solution is developed for the problem of diffraction of plane waves by a strip which is soft at one side and hard on the other. The related three-part boundary value problem is formulated into a “modified matrix Wiener-Hopf equation”. By using the known factorization of the kernel matrix through the Daniele-Khrapkov method, the modified matrix Wiener-Hopf equation is first reduced to a pair of coupled Fredholm integral equations of the second kind and then solved by iterations. An interesting feature of the present solution is that the classical Wiener-Hopf arguments yield unknown constants which can be determined by means of the edge conditions.