Abstract We report the discovery of a circumplanetary disk (CPD) candidate embedded in the circumstellar disk of the T Tauri star AS 209 at a radial distance of about 200 au (on-sky separation of 1.″4 from the star at a position angle of 161°), isolated via ¹³CO J = 2−1 emission. This is the first instance of CPD detection via gaseous emission capable of tracing the overall CPD mass. The CPD is spatially unresolved with a 117 × 82 mas beam and manifests as a point source in ¹³CO, indicating that its diameter is ≲14 au. The CPD is embedded within an annular gap in the circumstellar disk previously identified using ¹²CO and near-infrared scattered-light observations and is associated with localized velocity perturbations in ¹²CO. The coincidence of these features suggests that they have a common origin: an embedded giant planet. We use the ¹³CO intensity to constrain the CPD gas temperature and mass. We find that the CPD temperature is ≳35 K, higher than the circumstellar disk temperature at the radial location of the CPD, 22 K, suggesting that heating sources localized to the CPD must be present. The CPD gas mass is ≳0.095 M _Jup ≃ 30 M _⊕ adopting a standard ¹³CO abundance. From the nondetection of millimeter continuum emission at the location of the CPD (3σ flux density ≲26.4 μJy), we infer that the CPD dust mass is ≲0.027 M _⊕ ≃ 2.2 lunar masses, indicating a low dust-to-gas mass ratio of ≲9 × 10⁻⁴. We discuss the formation mechanism of the CPD-hosting giant planet on a wide orbit in the framework of gravitational instability and pebble accretion.