  We present MIRI MRS observations of the large, multi-gapped protoplanetary
disk around the T-Tauri star AS 209. The observations reveal hundreds of water
vapor lines from 4.9 to 25.5 $\mu$m towards the inner $\sim1$ au in the disk,
including the first detection of ro-vibrational water emission in this disk.
The spectrum is dominated by hot ($\sim800$ K) water vapor and OH gas, with
only marginal detections of CO$_2$, HCN, and a possible colder water vapor
component. Using slab models with a detailed treatment of opacities and line
overlap, we retrieve the column density, emitting area, and excitation
temperature of water vapor and OH, and provide upper limits for the observable
mass of other molecules. Compared to MIRI spectra of other T-Tauri disks, the
inner disk of AS 209 does not appear to be atypically depleted in CO$_2$ nor
HCN. Based on \textit{Spitzer IRS} observations, we further find evidence for
molecular emission variability over a 10-year baseline. Water, OH, and CO$_2$
line luminosities have decreased by factors 2-4 in the new MIRI epoch, yet
there are minimal continuum emission variations. The origin of this variability
is yet to be understood.
