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We combine Keck/HIRES radial velocities, imaging with HiCIAO/Subaru and theHubble Space Telescope, and absolute astrometry from Hipparcos and Gaia tomeasure a dynamical mass of $70 \pm 5$ Jupiter masses for the brown dwarfcompanion to Gl 229. Gl 229B was the first imaged brown dwarf to show clearsigns of methane in its atmosphere. Cooling models have been used to estimate amass in the range of 20 - 55 Jupiter masses, much lower than our measuredvalue. We argue that our high dynamical mass is unlikely to be due toperturbations from additional unseen companions or to Gl 229B being itself abinary, and we find no evidence of a previously claimed radial velocity planetaround Gl 229A. Future Gaia data releases will confirm the reliability of theabsolute astrometry, though the data pass all quality checks in both Hipparcosand Gaia. Our dynamical mass implies a very old age for Gl 229, in some tensionwith kinematic and activity age indicators, and/or shortcomings in brown dwarfcooling models. Gl 229B joins a small but growing list of T dwarfs with massesapproaching the minimum mass for core hydrogen ignition.

A Dynamical Mass of $70 \pm 5$ Jupiter Masses for Gliese 229B, the First T Dwarf