  New and existing photometry for the G0 Ia supergiant HD 18391 is analyzed in
order to confirm the nature of the variablity previously detected in the star,
which lies off the hot edge of the Cepheid instability strip. Small-amplitude
variability at a level of \Delta V = 0.016+-0.002 is indicated, with a period
of P=123.04+-0.06 d. A weaker second signal may be present at P=177.84+-0.18
with \Delta V = 0.007+-0.002, likely corresponding to fundamental mode
pulsation if the primary signal represents overtone pulsation (123.04/177.84 =
0.69). The star, with a spectroscopic reddening of E(B-V) = 1.02, is associated
with heavily-reddened B-type stars in its immediate vicinity that appear to be
outlying members of an anonymous young cluster centered ~10 arcmin to the west
and 1661+-73 pc distant. The cluster has nuclear and coronal radii of r_n=3.5
arcmin and R_c=14 arcmin, respectively, while the parameters for HD 18391
derived from membership in the cluster with its outlying B stars are consistent
with those implied by its Cepheid-like pulsation, provided that it follows the
semi-period-luminosity relation expected of such objects. Its inferred
luminosity as a cluster member is M_V=-7.76+-0.10, its age (9+-1)x10^6 years,
and its evolutionary mass ~19 M_{\sun}. HD 18391 is not a classical Cepheid,
yet it follows the Cepheid period-luminosity relation closely, much like
another Cepheid impostor, V810 Cen.
