The use of chevron braces for seismic strengthening of existing non-ductile reinforced concrete frames was investigated experimentally and numerically. Five strengthened and two as-built reference portal frame specimens were tested under a constant gravity load and increasing cyclic lateral displacement excursions. The test results indicated that steel brace retrofitting increased the lateral stiffness, strength and energy dissipation capacity significantly. Specimens strengthened with square hollow sections experienced significant strength degradation upon brace buckling, but a lateral strength of about 3.5 times the strength of bare frames was maintained up to a drift ratio of about 2% in all strengthened specimens. Non-linear cyclic analysis was conducted to simulate the behaviour of the test specimens. A satisfactory agreement with estimated and observed load-deformation responses was observed. Using test observations and simulation results, the deformation capacity limits of chevron brace strengthened deficient frames were discussed. A parametric study was conducted to investigate the effectiveness of the chevron brace retrofitting for different brace sizes and column axial loads. Finally, a non-linear time history analysis was performed for a building frame of a deficient existing four-storey three-bay reinforced concrete structure by using two chevron brace sections in this study.