We report the first detection in space of the single deuterated isotopologue of methyldiacetylene, CH 2 DC 4 H. A total of 12 rotational transitions, with J = 8-12 and K a = 0 and 1, were identified for this species in TMC-1 in the 31.0-50.4 GHz range using the Yebes 40m radio telescope. The observed frequencies allowed us to obtain, for the first time, the spectroscopic parameters of this deuterated isotopologue. We derived a column density of (5.5±0.2)×10 11 cm −2. The abundance ratio between CH 3 C 4 H and CH 2 DC 4 H is 24±2. This ratio is similar to that found for the CH 3 C 3 N/CH 2 DC 3 N analogue system, which is 22±2. We did not detect the deuterated species CH 3 C 4 D, which has already been observed in laboratory experiments. The detection of deuterated CH 3 C 4 H allows us to extend the discussion on the chemical mechanisms of deuterium fractionation at work in TMC-1 using a new gas-phase chemical model with multiply deuterated molecules. Introducing a possible deuterium exchange reaction between CH 3 CCH and atomic deuterium allows us to account for the CH 3 C 4 H/CH 2 DC 4 H abundance ratio.