Brittle behavior of substandard columns during earthquakes is among the most common reasons of structural failures. Because capacity design principles and ductile detailing concepts have not been properly adopted during the construction of a large portion of existing buildings, many of them require seismic retrofitting. Among other deficiencies, lack of sufficient ductility is generally the major problem. External fiber-reinforced polymer (FRP) jacketing of columns is a feasible and promising method to overcome the problem of insufficient column ductility in existing substandard buildings. However, experimental data on the behavior of full-scale substandard columns is scarce, particularly in the case of columns with extended rectangular cross sections. Consequently, current standards do not allow FRP jacketing for the enhancement of ductility against seismic actions when the cross-sectional aspect ratio is higher than 1.5. Therefore, in this study, the seismic performance of full-scale substandard columns with extended rectangular cross sections is investigated both experimentally and theoretically. The columns are tested before or after retrofitting with carbon fiber-reinforced polymer (CFRP) jacketing. The findings of the study clearly demonstrate the efficiency of FRP jacketing for substandard columns, even for the case of extended rectangular cross sections. Furthermore, nonlinear analyses of reference and FRP-retrofitted columns, and comparison of the results of analytical study with experimental data, revealed that FRP retrofit design methods recommended by the American Concrete Institute (ACI) and Turkish codes are quite conservative, leading to unfeasible retrofit options, whereas the Eurocode approach leads to more realistic and economical retrofit solutions.