  We present the discovery of a luminous X-ray transient, serendipitously
detected by Swift's X-ray Telescope (XRT) on 2020 February 5, located in the
nucleus of the galaxy SDSS J143359.16+400636.0 at z=0.099 (luminosity distance
$D_{\rm L}=456$ Mpc). The transient was observed to reach a peak luminosity of
$\sim10^{44}$ erg s$^{-1}$ in the 0.3--10 keV X-ray band, which was $\sim20$
times more than the peak optical/UV luminosity. Optical, UV, and X-ray
lightcurves from the Zwicky Transient Facility (ZTF) and Swift show a decline
in flux from the source consistent with $t^{-5/3}$, and observations with
NuSTAR and Chandra show a soft X-ray spectrum with photon index
$\Gamma=2.9\pm0.1$. The X-ray/UV properties are inconsistent with well known
AGN properties and have more in common with known X-ray tidal disruption events
(TDE), leading us to conclude that it was likely a TDE. The broadband spectral
energy distribution (SED) can be described well by a disk blackbody model with
an inner disk temperature of $7.3^{+0.3}_{-0.8}\times10^{5}$ K, with a large
fraction ($>40$%) of the disk emission up-scattered into the X-ray band. An
optical spectrum taken with Keck/LRIS after the X-ray detection reveals LINER
line ratios in the host galaxy, suggesting low-level accretion on to the
supermassive black hole prior to the event, but no broad lines or other
indications of a TDE were seen. The stellar velocity dispersion implies the
mass of the supermassive black hole powering the event is log($M_{\rm
BH}$/$M_{\odot}$)$=7.41\pm0.41$, and we estimate that at peak the Eddington
fraction of this event was $\sim$50%. This likely TDE was not identified by
wide-field optical surveys, nor optical spectroscopy, indicating that more
events like this would be missed without wide-field UV or X-ray surveys.
