Tomato gray leaf spot (TGLS), caused by Stemphylium spp., is a common disease leading to significant economic losses in tomato production. Pydiflumetofen is a novel succinate dehydrogenase inhibitor (SDHI) fungicide that has been registered for TGLS management. To evaluate the susceptibility of S. lycopersici to pydiflumetofen in tomato-producing regions of Hebei Province, we determined the sensitivity of 212 S. lycopersici isolates using mycelial growth inhibition. The sensitivity distribution exhibited a multimodal pattern. Resistance to pydiflumetofen was observed in some field isolates, with highly resistant isolates being identified in Chengde, Hengshui, and Tangshan. After removing outliers, the baseline sensitivity of S. lycopersici to pydiflumetofen was established, with a mean EC50 value of 1.0400 ± 0.0515 μg/mL. Sequence analysis revealed point mutations only in SdhC (SdhCS73P, SdhCG79R, SdhCH134R, SdhCS135R) among the resistant isolates. No significant differences were observed between certain resistant isolates (FQSL1-10 and FQSL1-14) and the sensitive isolates in temperature adaptability, mycelial growth rate, or pathogenicity. These results suggest that pydiflumetofen has high activity against TGLS, but integrated fungicide application is necessary for delaying resistance evolution in TGLS management.