To generate bright water-window (WW) soft x rays (2.3–4.4 nm), gold slab targets were irradiated with laser pulses (1064 nm, 7 ns, 1 J). Emission spectroscopy showed that the introduction of low-pressure nitrogen enhanced the soft x-ray yield emitted from the laser-produced Au plasma. The intensity of the WW x-ray transported in a 400-Pa N 2 atmosphere from the laser-produced plasma increased by 3.8 times over that in vacuum. Considering a strong x-ray absorption, the x-ray yield emitted directly from the Au plasma in the N 2 gas was evaluated to be 13 times higher than that in vacuum. Although similar measurements were made for various gases, only N 2 gas causes an increase in a soft x-ray yield. The processes leading to this enhancement mechanism were revealed by using hydrodynamic simulation and atomic structure codes.