High-performance hydrogen sensors with excellent mechanical flexibility and durability have been fabricated on thin plastic sheets with the use of high-quality semiconducting single-walled carbon nanotubes decorated with discrete Pd nanoparticles. These sensors exhibit sensing performance much higher than that of the traditional ones built with precious pure palladium structures on rigid substrates. For example, the sensitivity of the typical flexible sensors is in the range of 100-150% (based on resistance change of the sensors) for 0.1% hydrogen in dry air at room temperature. The response times are typically less than 15 s for 1% hydrogen, and the sensors can be completely recovered within 5 min in the air without hydrogen. The flexible sensors can detect hydrogen with concentrations as low as 100 ppm (0.01%). Experimental results indicate that the as-fabricated devices operate well even when they are bent to a curved geometry with bending radii down to 2 mm and after they are treated with 1000 times of bending/relaxing cycles. The sensing mechanism is also discussed semiquantitatively on the basis of the theory of solid-state physics and Langmuir adsorption isotherm theory. The conclusion is consistent with the characteristic results of the as-fabricated sensors. These flexible hydrogen sensors could find immediate applications for detecting hydrogen leakage in various systems with demanding light weight, mechanical flexibility, and high sensitivity.