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Mature super-Earths and sub-Neptunes are predicted to be $\simeq$Jovianradius when younger than 10 Myr. Thus, we expect to find 5-15$R_\oplus$ planetsaround young stars even if their older counterparts harbor none. We report thediscovery and validation of TOI 1227 b, a $0.85\pm0.05R_J$ (9.5$R_\oplus$)planet transiting a very low-mass star ($0.170\pm0.015M_\odot$) every 27.4days. TOI~1227's kinematics and strong lithium absorption confirm it is amember of a previously discovered sub-group in the Lower Centaurus Crux OBassociation, which we designate the Musca group. We derive an age of 11$\pm$2Myr for Musca, based on lithium, rotation, and the color-magnitude diagram ofMusca members. The TESS data and ground-based follow-up show a deep (2.5\%)transit. We use multiwavelength transit observations and radial velocities fromthe IGRINS spectrograph to validate the signal as planetary in nature, and weobtain an upper limit on the planet mass of $\simeq0.5 M_J$. Because such largeplanets are exceptionally rare around mature low-mass stars, we suggest thatTOI 1227 b is still contracting and will eventually turn into one of the morecommon $<5R_\oplus$ planets.

TESS Hunt for Young and Maturing Exoplanets (THYME) VI: an 11 Myr giant planet transiting a very low-mass star in Lower Centaurus Crux