We present the first radio/submillimeter detection of monodeuterated methane (CH$_3$D) in Titans atmosphere, using archival data from of the Atacama Large Millimeter/submillimeter Array (ALMA). The $J_K=2_1-1_1$ and $J_K=2_0-1_0$ transitions at 465.235 and 465.250 GHz ($sim0.644$ mm) were measured at significance levels of $4.6sigma$ and $5.7sigma$, respectively. These two lines were modeled using the Non-linear optimal Estimator for MultivariatE spectral analySIS (NEMESIS) radiative transfer code to determine the disk-averaged CH$_3$D volume mixing ratio = $6.157times10^{-6}$ in Titans stratosphere (at altitudes $gt130$ km). By comparison with the CH$_4$ vertical abundance profile measured by Cassini-Huygens mass spectrometry, the resulting value for D/H in CH$_4$ is $(1.033pm0.081)times10^{-4}$. This is consistent with previous ground-based and in-situ measurements from the Cassini-Huygens mission, though slightly lower than the average of the previous values. Additional CH$_3$D observations at higher spatial resolution will be required to determine a value truly comparable with the Cassini-Huygens CH$_4$ measurements, by measuring CH$_3$D with ALMA close to Titans equator. In the post-Cassini era, spatially resolved observations of CH$_3$D with ALMA will enable the latitudinal distribution of methane to be determined, making this an important molecule for further studies.
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