ABSTRACTPlants regulate gas exchange with the environment and modulate transpirational water flow through guard cells, which set the aperture of the stomatal pores. External and internal stimuli are detected by guard cells and integrated into a signalling network that modulate turgor pressure and, hence, pore size. Pathogen‐associated molecular patterns are among the stimuli that induce stomatal closure, to prevent pathogen entry through the pores, and this response, also referred to as stomatal immunity, is one of the hallmarks of PAMP‐triggered immunity. While reactive oxygen species (ROS)‐mediated signalling plays a key role in stomatal immunity, also the gasotransmitter hydrogen sulphide (H2S) interacts with key components of the guard cell signalling network to induce stomatal closure. While the role of H2S, produced by the main cytosolic source L‐cysteine desulfhydrase 1, has been already investigated, there are additional enzymatic sources that synthesize H2S in different subcellular compartments. Their function has remained enigmatic, however. In this work, we elucidate the involvement of the mitochondrial H2S source, β‐cyanoalanine synthase CAS‐C1, on stomatal immunity induced by the bacterial PAMP flagellin (flg22). We show that cas‐c1 plants are impaired to induce flg22‐triggered stomatal closure and apoplastic ROS production, while they are more susceptible to bacterial surface inoculation. Moreover, mitochondrial H2S donor AP39 induced stomatal closure in an RBOHD‐dependent manner, while depletion of endogenous H2S, impaired RBOHD‐mediated apoplastic ROS production. In addition, pharmacological disruption of mitochondrial electron transport chain activity, affected stomatal closure produced by flg22, indicating its participation in the stomatal immunity response. Our findings add evidence to the emerging realization that intracellular organelles play a decisive role in orchestrating stomatal signalling and immune responses and suggest that mitochondrial‐derived H2S is an important player of the stomatal immunity signalling network.