  [abridged] ALMA observations of dust in protoplanetary disks are revealing
the existence of sub-structures such as rings, gaps and cavities. Such
morphology are expected to be the outcome of dynamical interaction between the
disk and planets. However, other mechanisms are able to produce similar dust
sub-structures. A solution is to look at the perturbation induced by the planet
to the gas surface density and/or to the kinematics. In the case of the disk
around AS 209, a prominent gap has been reported in the surface density of CO
at $r \sim 100\,$au. Recently, Bae et al. (2022) detected a localized velocity
perturbation in the $^{12}$CO $J=2-1$ emission along with a clump in $^{13}$CO
$J=2-1$ at nearly 200 au, interpreted as a gaseous circumplanetary disk. We
report a new analysis of ALMA archival observations of $^{12}$CO and $^{13}$CO
J=2-1. A clear kinematics perturbation (kink) is detected in multiple channels
and over a wide azimuth range in both dataset. We compared the observed
perturbation with a semi-analytic model of velocity perturbations due to
planet-disk interaction. The observed kink is not consistent with a planet at
200\,au as this would require a low gas disk scale height ($< 0.05$) in
contradiction with previous estimate ($h/r \sim 0.118$ at $r = 100$ au). When
we fix the disk scale height to 0.118 (at $r = 100$ au) we find instead that a
planet of 3-5 M$_{\rm Jup}$ at 100 au induces a kinematics perturbation similar
to the observed one. Thus, we conclude that a giant protoplanet orbiting at $r
\sim 100\,$au is responsible of the large scale kink as well as of the
perturbed dust and gas surface density previously detected. The position angle
of the planet is constrained to be between 60$^{\circ}$-100$^{\circ}$. Future
observations with high contrast imaging technique in the near- and mid-
infrared are needed to confirm the presence and position of such a planet.
