Studying the gas content of post-starburst (PSB) galaxies can providevaluable clues regarding the process of fast quenching. Although previous workshave studied the molecular gas content of PSBs, only a handful of HImeasurements exist. Here, we present new Five hundred metre Aperture SphericalTelescope (FAST) 21cm observations of 44 PSBs, leading to 42 detections orsensitive upper limits of HI, which we combine with 26 archival measurements,for a total sample of 68 PSB MHI measurements. HI is detected in 57/68galaxies, with HI masses ranging from MHI ~10^8.5 up to 10^10 Msun and gasfractions (fHI = MHI/M*) from a few percent up to almost 30 percent.Post-starbursts therefore retain ample atomic gas reservoirs, despite no longerforming stars. By comparing with a stellar mass-matched sample of star-forminggalaxies in xGASS, we find that PSBs have, on average, gas fractions lower by~0.2-0.4 dex, consistent with a mild reduction compared with their progenitorpopulation. However, PSBs show a diversity of HI properties; about half have HIgas masses within the expected scatter of the star-forming population with theremaining 50 per cent up to a factor of 10 more gas-poor. Compared withgalaxies in the green valley, about two thirds of PSBs have gas fractionswithin the expected range, with the remaining third up to a factor of 10 moregas-rich. Our results demonstrate that quenching in PSBs is not the result ofwholesale removal of the atomic gas reservoir and that the population hasatomic gas fractions that span the range from star-forming to green valleygalaxies. We find no correlation between HI gas mass and time since burst; evengalaxies a Gyr past their burst can remain HI-normal. The significant gasreservoirs remaining in many PSBs leaves open the possibility for futurerekindling of star formation.