ABSTRACT We report the first detection in the JK = 10–00 rotational transition line of CH3D towards three Class 0/I proto-brown dwarfs (proto-BDs) from IRAM 30-m observations. Assuming a rotational temperature of 25 K, the CH3D abundances (relative to H2) are in the range of (2.3–14.5) × 10−7. The CH4 abundances derived from the CH3D abundances and assuming the DCO+/HCO+ ratios are in the range of (0.05–4.8) × 10−5. The gas-phase formation of CH3D via CH2D+ is enhanced at high densities of 108–1010 cm−3 and our observations are likely probing the innermost dense and warm regions in proto-BDs. Thermal and/or non-thermal desorption can return the CH3D and CH4 molecules formed at an early stage on grain surfaces to the gas-phase. The gas-phase abundances are indicative of warm carbon-chain chemistry in proto-BDs where carbon-chain molecules are synthesized in a lukewarm (∼20–30 K) region close to the central source.