The in-plane seismic performance of old unreinforced brick masonry walls strengthened with stainless-steel twisted bars and conventional steel bars (usually used in concrete reinforcing) was investigated in the present study. For this purpose, a total of seventeen masonry wallettes reproducing masonry walls of traditional Lisbon buildings from the 1930s–1950s (known as “placa” buildings) were constructed. The specimens were assembled using original bricks from demolished old buildings and a sand-cement bedding mortar with the same proportions as reported in building design documents of the time. Out of the fourteen wallettes tested in diagonal compression, three were unreinforced, and eleven were strengthened in different layouts. Additionally, axial compression tests were performed on the other three unreinforced masonry wallettes. The tested specimens were reproduced with numerical models calibrated based on the experimental results. The experimental and numerical results of the reinforced specimens were compared to the unreinforced specimens to draw conclusions on the effectiveness of the strengthening solutions when applied to masonry walls of the typology under study. The most important result of the strengthening was an important increase in the ductility of the specimens that were originally brittle. In addition, in some cases a slight increase of shear strength was also observed.