This paper presents the results of a numerical study about the performance of a new timber retrofit solution for unreinforced masonry buildings in seismic regions. The proposed retrofit consists of timber frames fastened to the internal surface of masonry piers, with oriented-strand boards nailed to the frames. This technique aims at improving both the in-plane and out-of-plane capacities of masonry piers as well as wall-to-diaphragm connections. Quasi-static cyclic shear-compression tests were first conducted on two isolated piers to assess the in-plane response improvement. Dynamic shake-table tests were then performed on two full-scale building specimens representing the end-unit of a cavity-wall terraced house, in bare and retrofitted conditions, where also the timber diaphragms were stiffened. Numerical models were built and calibrated against the experimental data adopting an equivalent-frame approach, where masonry members were discretized into multiple nonlinear macroelements while timber components were represented by equivalent elasto-plastic truss elements. Results in terms of hysteretic lateral force-displacement responses from nonlinear dynamic analyses are discussed to validate the proposed modeling strategy.