International audienceThe advent of X-ray free-electron lasers has granted researchers an unprecedented access to theultrafast dynamics of matter on the nanometre scale(1-3). Aside from being compact, seededplasma-based soft X-ray lasers (SXRLs) turn out to be enticing as photon-rich(4) sources (up to 10(15)per pulse) that display high-quality optical properties(5,6). Hitherto, the duration of these sources waslimited to the picosecond range(7), which consequently restricts the field of applications. This bottleneckwas overcome by gating the gain through ultrafast collisional ionization in a high-density plasmagenerated by an ultraintense infrared pulse (a few 10(18) W cm(-2)) guided in an optically pre-formedplasma waveguide. For electron densities that ranged from 3 x 10(18) cm(-3) to 1.2 x 10(20) cm(-3), thegain duration was measured to drop from 7 ps to an unprecedented value of about 450 fs, which pavesthe way to compact and ultrafast SXRL beams with performances previously only accessible inlarge-scale facilities