DnaJ proteins, constituents of the heat shock protein system, protect cells from environmental stresses as HSP-70 molecular co-chaperones. Here, we demonstrated that a pathogen-induced AdDjSKI, a serine-rich DnaJ III protein plays an important role in the stability of photosystem II in response to heat stress. Under high temperatures, transplastomic tomato plants expressing the AdDjSKI gene showed increased levels of total soluble proteins, better growth, higher chlorophyll content, lower malondialdehyde (MDA) accumulation and reduced Photosystem (PS) II photoinhibition compared to the wildtype (WT) control plants. Interestingly, the transplastomic plants showed higher D1 protein levels under high temperatures compared to the WT plants suggesting that the overexpression of AdDjSKI in the plastids played a key role in the protection of PSII probably because of its chaperone activity. Additionally, the transplastomic plants showed relatively lower accumulation of superoxide radical (O 2 •-) and H2O2 levels in comparison with the WT plants possibly due to higher ascorbate peroxidase (APX) and superoxide dismutase (SOD) activities, which was due to an increase in related gene expression under heat stress. This study also reveals the chloroplast expression of AdDjSKI in tomato play a critical role in fruit yield, primarily through a combination of delayed senescence and enhanced carbon assimilation.