We report the first detection in space of the two doubly deuterated isotopologues of methyl acetylene. The species CHD CCH and CH DCCD were identified in the dense core L483 through nine and eight, respectively, rotational lines in the 72-116 GHz range using the IRAM 30m telescope. The astronomical frequencies observed here were combined with laboratory frequencies from the literature measured in the 29-47 GHz range to derive more accurate spectroscopic parameters for the two isotopologues. We derive beam-averaged column densities of (2.7 ± 0.5) × 10 cm for CHD CCH and (2.2 ± 0.4) × 10 cm for CH DCCD, which translate to abundance ratios CH CCH/CHD CCH = 34 ± 10 and CH CCH/CH DCCD = 42 ± 13. The doubly deuterated isotopologues of methyl acetylene are only a few times less abundant than the singly deuterated ones, concretely around 2.4 times less abundant than CH CCD. The abundances of the different deuterated isotopologues with respect to CH CCH are reasonably accounted for by a gas-phase chemical model in which deuteration occurs from the precursor ions C H D and C H D , when the ortho-to-para ratio of molecular hydrogen is sufficiently low. This points to gas-phase chemical reactions, rather than grain-surface processes, as responsible for the formation and deuterium fractionation of CH CCH in L483. The abundance ratios CH DCCH/CH CCD = 3.0 ± 0.9 and CHD CCH/CH DCCD = 1.25 ± 0.37 observed in L483 are consistent with the statistically expected values of three and one, respectively, with the slight overabundance of CHD CCH compared to CH DCCD being well explained by the chemical model.