  The fast blue optical transients (FBOTs) are a new population of
extragalactic transients of unclear physical origin. A variety of mechanisms
have been proposed including failed supernova explosion, shock interaction with
a dense medium, young magnetar, accretion onto a compact object, and stellar
tidal disruption event, but none is conclusive. Here we report the discovery of
a possible X-ray quasi-periodicity signal with a period of $\sim$250 second (at
a significance level of 99.76%) in the brightest FBOT AT2018cow through the
analysis of XMM-Newton/PN data. The signal is independently detected at the
same frequency in the average power density spectrum from data taken from the
Swift telescope, with observations covering from 6 to 37 days after the optical
discovery, though the significance level is lower (94.26%). This suggests that
the QPO frequency may be stable over at least 1.1$\times$ 10$^{4}$ cycles.
Assuming the $\sim$250 second QPO to be a scaled-down analogue of that
typically seen in stellar mass black holes, a black hole mass of
$\sim10^{3}-10^{5}$ solar masses could be inferred. The overall X-ray
luminosity evolution could be modeled with the stellar tidal disruption by a
black hole of $\sim10^4$ solar masses, providing a viable mechanism to produce
AT2018cow. Our findings suggest that other bright FBOTs may also harbor
intermediate-mass black holes.
