  We present simultaneous HST WFC3 + Spitzer IRAC variability monitoring for
the highly-variable young ($\sim$20 Myr) planetary-mass object PSO J318.5-22.
Our simultaneous HST + Spitzer observations covered $\sim$2 rotation periods
with Spitzer and most of a rotation period with HST. We derive a period of
8.6$\pm$0.1 hours from the Spitzer lightcurve. Combining this period with the
measured $v sin i$ for this object, we find an inclination of 56.2$\pm
8.1^{\circ}$. We measure peak-to-trough variability amplitudes of
3.4$\pm$0.1$\%$ for Spitzer Channel 2 and 4.4 - 5.8$\%$ (typical 68$\%$
confidence errors of $\sim$0.3$\%$) in the near-IR bands (1.07-1.67 $\mu$m)
covered by the WFC3 G141 prism -- the mid-IR variability amplitude for PSO
J318.5-22 one of the highest variability amplitudes measured in the mid-IR for
any brown dwarf or planetary mass object. Additionally, we detect phase offsets
ranging from 200--210$^{\circ}$ (typical error of $\sim$4$^{\circ}$) between
synthesized near-IR lightcurves and the Spitzer mid-IR lightcurve, likely
indicating depth-dependent longitudinal atmospheric structure in this
atmosphere. The detection of similar variability amplitudes in wide spectral
bands relative to absorption features suggests that the driver of the
variability may be inhomogeneous clouds (perhaps a patchy haze layer over thick
clouds), as opposed to hot spots or compositional inhomogeneities at the
top-of-atmosphere level.
