Abstract We report the fabrication of a fast response hydrogen gas sensor based on palladium decorated single-walled carbon nanotube (Pd-SWNT) Langmuir–Blodgett (LB) film. To construct the sensor device, a monolayer of aligned SWNT film is deposited on an Au inter-digital transducer (IDT) by the LB technique at a surface pressure of 25mN/m, followed by the electrochemical reductive deposition of palladium nanoparticles on the SWNT LB films. The Pd-SWNT sensor showed a detection ability of 0.025–2.5% (v/v) of hydrogen in nitrogen (N2) atmosphere at room temperature and demonstrated a reversible detection process and very fast response time. We analyzed the effect of the palladium acetate concentration used for synthesis of the Pd nanoparticles on the conductance change of the Pd-SWNT sensor. A palladium acetate concentration of 10μM yielded the highest response of hydrogen detection whereas higher concentrations led to a change of the electron transport property of the films from semiconducting to metallic and consequently decreased the performance of the sensor. The study demonstrates that the Pd-SWNT sensor can easily be extended to detect other gaseous molecules.