  We report the discovery of a 20-kpc-sized H{\alpha} emission in SDSS
J083803.68+540642.0, a ringed dwarf galaxy (M$_V$ = -17.89 mag) hosting an
accreting intermediate-mass black hole at z=0.02957. Analysis of the HST images
indicates that it is an early-type galaxy with a featureless low-surface
brightness disk ({\mu}0 = 20.39 mag arcsec$^{-2}$ in the V band) and a
prominent, relatively red bulge (V - I = 2.03, Re = 0.28 kpc or 0."48) that
accounts for ~81% of the total light in the I band. A circumgalactic ring of a
diameter 16 kpc is also detected, with a disperse shape on its south side. The
optical emission lines reveal the nucleus to be a broad-line LINER. Our MMT
longslit observation indicates that the kinematics of the extended H{\alpha}
emission is consistent with a rotational gaseous disk, with a mean blueshifted
velocity of 162 km s$^{-1}$ and mean redshifted velocity of 86 km s$^{-1}$ .
According to our photoionization calculations, the large-scale H{\alpha}
emission is unlikely to be powered by the central nucleus or by hot evolved
(post-AGB) stars interspersed in the old stellar populations, but by in situ
star formation; this is vindicated by the line-ratio diagnostic of the extended
emission. We propose that both the ring and large-scale H{\alpha}-emitting gas
are created by the tidal accretion in a collision-and then merger-with a
gas-rich galaxy of a comparable mass.
