We report on the observation of resonant intersubband second‐harmonic generation in asymmetric GaAs/AlGaAs quantum wells using a cw orQ‐switched tunable CO2laser as the pumping source. The dependence of the second‐harmonic intensity with the pump photon wavelength is presented for the first time. A Lorentzian‐like second‐harmonic line shape is found with a maximum at 10.9 &mgr;m and a linewidth of 0.4 &mgr;m (4.1 meV). These results are in good agreement with theoretical predictions. The expected quadratic dependence of the second‐harmonic conversion efficiency with pump intensity is well verified for intensities up to 150 kW/cm2. The calibrated second‐harmonic power reaches 0.13 &mgr;W for a cw pump power of 0.8 W. The value of 7.2×10−7m/V deduced for the second‐order nonlinear susceptibility is about 1900 times greater than that found in bulk GaAs.