  We present X-ray imaging spectroscopy of one of the weakest active region
(AR) microflares ever studied. The microflare occurred at $\sim$11:04 UT on
2018 September 9 and we studied it using the Nuclear Spectroscopic Telescope
ARray (NuSTAR) and the Solar Dynamic Observatory's Atmospheric Imaging Assembly
(SDO/AIA). The microflare is observed clearly in 2.5-7 keV with NuSTAR and in
Fe XVIII emission derived from the hotter component of the 94 $\unicode{x212B}$
SDO/AIA channel. We estimate the event to be three orders of magnitude lower
than a GOES A class microflare with an energy of 1.1$\times$10$^{26}$ erg. It
reaches temperatures of 6.7 MK with an emission measure of 8.0$\times$10$^{43}$
cm$^{-3}$. Non-thermal emission is not detected but we instead determine upper
limits to such emission. We present the lowest thermal energy estimate for an
AR microflare in literature, which is at the lower limits of what is still
considered an X-ray microflare.
