Corrosion is one of the major problems for the existing structures and may cause significant decrease in drift capacity and strength of RC (reinforced concrete) columns due to reduction of effective cross-section of reinforcing bars, alteration of bond characteristics between reinforcing bars and concrete, and cover cracking along the reinforcing bars. Therefore, rehabilitation of corrosion-damaged columns is extremely important, particularly in seismic regions and if a substandard construction methodology is used. In this paper, an experimental work is presented on the seismic performance of rehabilitated/retrofitted substandard RC columns, which were damaged due to corrosion of reinforcing bars. For this purpose, four substandard columns were tested under high axial load and reversed cyclic lateral loads. The columns were constructed with extremely low quality concrete and plain round bars. The spacing and details of transverse reinforcing bars did not comply with the code regulations as well. Then, the specimens, except the reference one, were subjected to accelerated corrosion process. One of the corrosion-damaged specimens was tested before rehabilitation/retrofitting procedure, while the other two specimens were tested after rehabilitation/retrofitting procedure. The test results clearly demonstrated the efficiency of the applied rehabilitation/retrofitting procedure, both in terms of strength and ductility. Furthermore, analytical predictions about strength and failure modes of the specimens are compared with experimental findings.