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 Hubble Space Telescope 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, R-e = 0.28 kpc or 0 48) that accounts for approximate to 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.