The formation of planets occurs within protoplanetary disks surrounding young stars, resulting in perturbation of the gas and dust surface densities. Here, we report the first evidence of spatially resolved gas surface density ($\Sigma g$) perturbation towards the AS~209 protoplanetary disk from the optically thin C$\mathrm{<mrow\; data-mjx-texclass="ORD">18</mrow>}$O ($J=2-1$) emission. The observations were carried out at 1.3~mm with ALMA at a spatial resolution of about 0.3$\backslash arcsec$ $\times$ 0.2$\backslash arcsec$ (corresponding to $\sim$ 38 $\times$ 25 au). The C$\mathrm{<mrow\; data-mjx-texclass="ORD">18</mrow>}$O emission shows a compact ($\le$60 au),centrallypeakedemissionandanouterringpeakingat140 au,consistentwiththatobservedinthecontinuumemissionand,itsazimuthallyaveragedradialintensityprofilepresentsadeficitthatisspatiallycoincidentwiththepreviouslyreporteddustmap.Thisdeficitcanonlybereproducedwithourphysico-thermochemicaldiskmodelbylowering$\Sigma_{gas}$ by nearly an order of magnitude in the dust gaps. Another salient result is that contrary to C$\mathrm{<mrow\; data-mjx-texclass="ORD">18</mrow>}$O, the DCO$\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$ ($J=3-2$) emission peaks between the two dust gaps. We infer that the best scenario to explain our observations (C$\mathrm{<mrow\; data-mjx-texclass="ORD">18</mrow>}$O deficit and DCO$\mathrm{<mrow\; data-mjx-texclass="ORD">+</mrow>}$ enhancement) is a gas perturbation due to forming-planet(s), that is commensurate with previous continuum observations of the source along with hydrodynamical simulations. Our findings confirm that the previously observed dust gaps are very likely due to perturbation of the gas surface density that is induced by a planet of at least 0.2~M in formation. Finally, our observations also show the potential of using CO isotopologues to probe the presence of saturn mass planet(s).