  The existence of intermediate-mass black holes (IMBHs) is crucial for
understanding various astrophysical phenomena, yet their existence remains
elusive, except for the LIGO-Virgo detection. We report the discovery of a
high-velocity star J0731+3717, whose backward trajectory about 21 Myr ago
intersects that of globular cluster M15 within the cluster tidal radius. Both
its metallicity [Fe/H] and its alpha-to-iron abundance ratio [$\alpha$/Fe] are
consistent with those of M15. Furthermore, its location falls right on the
fiducial sequence of the cluster M15 on the color-absolute magnitude diagram,
suggesting similar ages. These support that J0731+3717 is originally associated
with M15 at a confidence level of $5.4\sigma$. We find that such a
high-velocity star ($V_{\rm ej} = 548^{+6}_{-5}$ km s$^{-1}$) was most likely
tidally ejected from as close as one astronomical unit to the center of M15,
confirming an IMBH ($\ge 100 M_{\odot}$ with a credibility of 98%) as the
exclusive nature of the central unseen mass proposed previously.
