We experimentally demonstrate the importance of two‐dimensional carrier confinement for picosecond dynamics in gain‐switched quantum well lasers, measuring time‐resolved spectra of gain‐switched quantum well (QW) lasers with coupled QWs and uncoupled QWs. The result indicates that an extremely short pulse (<2 ps) is generated in the uncoupled QW lasers. On the other hand, the pulse duration is about 10 ps in the coupled QW lasers in which the two‐dimensional confinement effect is significantly reduced owing to the miniband formation. These results demonstrate the importance of the two‐dimensional confinement of carriers for the short pulse generation in the semiconductor lasers. The measured dynamic spectral shift of the gain‐switched QW lasers also confirm the significant role of the two‐dimensional carrier confinement.