  We present Hubble Space Telescope (HST) observations of the field of the Type
IIb supernova (SN) 2011dh in M51 performed at ~1161 rest-frame days after
explosion using the Wide Field Camera 3 and near-UV filters F225W and F336W. A
star-like object is detected in both bands and the photometry indicates it has
negative (F225W - F336W) color. The observed object is compatible with the
companion of the now-vanished yellow supergiant progenitor predicted in
interacting binary models. We consider it unlikely that the SN is undergoing
strong interaction and thus estimate that it makes a small contribution to the
observed flux. The possibilities of having detected an unresolved light echo or
an unrelated object are briefly discussed and judged unlikely. Adopting a
possible range of extinction by dust, we constrain parameters of the proposed
binary system. In particular, the efficiency of mass accretion onto the binary
companion must be below 50%, if no significant extinction is produced by newly
formed dust. Further multiband observations are required in order to confirm
the identification of the object as the companion star. If confirmed, the
companion star would already be dominant in the UV/optical regime, so it would
readily provide a unique opportunity to perform a detailed study of its
properties.
