Herein, the lasing threshold and gain characteristics of ultraviolet‐C optically pumped edge‐emitting lasers with thick single‐quantum‐well (SQW) active regions are investigated by the variable‐stripe length method. Positive net modal gain is observed in lasers with AlGaN‐based SQWs with thicknesses up to 12 nm. The lasers show a reduction of the threshold power density with increasing SQW thickness, with the lowest threshold of 1.3 MW cm⁻² achieved for a 9 nm SQW laser. The high gain and low threshold in lasers with thick quantum wells are attributed to lasing from excited states, where the polarization fields are screened by the carriers in the fundamental state and the barriers, thus recovering larger electron and hole wavefunctions overlap. These findings are supported by k · p simulations, and for a 9 nm SQW, the calculation predicts a contribution of the fundamental transition to the gain of 75% for non‐resonant optical excitation and below 1% for resonant optical or electrical excitation.