Highly sensitive and reliable hydrogen detection is prerequisite for the large‐scale implementation of green energy hydrogen. It remains a tough challenge to get a highly selective H2 sensor to low concentration H2 gas with low working temperature. In this work, high performance flexible hydrogen sensor using ultrathin SnO2 film decorated by Pd nanoparticles (NPs) on polyimide (PI) was developed based on versatile atomic layer deposition (ALD) and cluster beam deposition (CBD). The influence of Pd NPs loading amount and operating temperature on the performance of hydrogen sensors was studied. The sensors obtain an extremely high response over 20000 to 30 ppm H2 at the relatively low operating temperature of 75–125 °C. The Pd NPs@SnO2/PI sensor achieves a wide detection range of 0.1‐5000 ppm with the limit of detection (LOD) of 100 ppb at 125 °C. A low LOD of 250 ppb can still be achieved at room temperature. The sensors also exhibit outstanding sensing selectivity for hydrogen over interfering gases and excellent mechanical resistance after 2000 bending cycles. The mechanism for improved sensing performance of ALD ultrathin film with CBD‐derived noble metal NPs decoration may be a feasible strategy for the construction of highly sensitive, selective, and flexible hydrogen sensors.