Abstarct: Abstract Hollow clay block masonry infills are among the most common non-structural components in reinforced concrete (RC) buildings in Iran. They are widely used due to their light weight and architectural advantages; however, they are highly vulnerable to seismic actions. Earthquake experience has shown that the failure of such infills especially under the combined effects of in-plane and out-of-plane loads can lead to considerable human and economic losses. Therefore, investigating their seismic behavior and proposing simple and cost-effective strengthening methods is of great importance. This dissertation experimentally investigates the seismic performance of masonry infill walls in RC frxames, including three configurations: bare frxame, frxame with hollow clay block infill, and frxame with strengthened infill. The strengthening technique involved the use of alkali-resistant glass fibers combined with a cement–sand plaster coating. All specimens were constructed at half-scale (1:2) and tested under quasi-static cyclic in-plane and out-of-plane loading. To evaluate interaction effects, some specimens were first subjected to in-plane loading followed by out-of-plane loading. The main investigated parameters included initial stiffness, load-carrying capacity, ductility, and cracking pattern. The results demonstrated that the proposed strengthening method enhanced the load-bearing capacity of the infilled frxame by more than 38% under in-plane loading and by over 170% under out-of-plane loading. For frxames that had sustained prior in-plane damage, the out-of-plane strength decreased by more than 50%. The proposed strengthening approach, which is both simple and economical, not only controlled crack propagation but also significantly improved the strength, stiffness, and ductility of the infill walls.