  We systematically investigate the mid-infrared (MIR; $\lambda>3 ~\mu$m) time
variability of uniformly selected $\sim800$ massive young stellar objects
(MYSOs) from the Red MSX Source (RMS) survey. Out of the 806 sources, we obtain
reliable 9-year-long MIR magnitude variability data of 331 sources at the
3.4~$\mu$m (W1) and 4.6~$\mu$m (W2) bands by cross-matching the MYSO positions
with ALLWISE and NEOWISE catalogs. After applying the variability selections
using ALLWISE data, we identify 5 MIR-variable candidates. The light curves
show various classes, with the periodic, plateau-like, and dipper features. Out
of the obtained two color-magnitude diagram of W1 and W1$-$W2, one shows "bluer
when brighter and redder when fainter" trends in variability, suggesting change
in extinction or accretion rate. Finally, our results show that
G335.9960$-$00.8532 (hereafter, G335) has a periodic light curve, with a
$\approx 690$-day cycle. Spectral energy density model fitting results indicate
that G335 is a relatively evolved MYSO; thus, we may be witnessing the very
early stages of a hyper- or ultra-compact HII region, a key source for
understanding MYSO evolution.
