  We report on the discovery of an infrared counterpart to the X-ray transient
XTE J1720-318 on 2003 January 18, nine days after an X-ray outburst, and the
infrared light curve during the first 130 days after the outburst. The infrared
light curve shows a decline of about 1.2 mag from the peak magnitude of Ks
about 15.3 over the observation period, and a secondary maximum, about 40 days
after the outburst. Another small increase in the flux was also recorded about
20 days after the outburst. These increases were also detected in the X-ray
light curve. The J H Ks colors are consistent with an X-ray irradiated
accretion disk suffering an extinction of Av about 8, which is also inferred
from its X-ray spectrum and the extinction map constructed from far-infrared
dust emission of this line of sight. These J, H, and Ks observations
demonstrate that useful data can be obtained even for such an object, which
suffers heavy optical extinction, possibly located beyond the Galactic center.
