The study presented herein proposes a retrofit method aimed at reducing the seismic vulnerability of reinforced concrete (RC) frame structures. The method consists in the replacement of the existing masonry infills with timber structural panels made of Cross Laminated Timber (CLT) fixed to the concrete frame by using a timber subframe and dissipative metal dowel-type fasteners. The first part of the research was carried out by performing nonlinear static analyses of finiteelement (FE) models of bare, masonry infilled and retrofitted single-storey single-bay frames. A large number of configurations was analysed considering different original conditions (e.g. in terms of geometrical characteristics, mechanical properties and loading) and several retrofit implementation approaches. Special attention was paid to the improvement of the seismic response of the beam-column joints, that represent a well-known structural vulnerability of existing concrete frame-buildings. The analysis results permitted to define a set of “general rules” to guide the implementation of the retrofit method depending on the characteristics of the original structure. Using these design rules, the proposed solution was then applied to the FE models of three case-study buildings, located in Italy and built in the period from 1950 to 1990. By comparing the seismic response of the pre- and post-intervention structures, it was observed that the proposed system could significantly improve the structural behaviour of the buildings, favouring the development of ductile mechanisms and reducing the vulnerability of the beam-column joints.