Chloroplast biogenesisdevelopment are coordinated by both endogenous factorsenvironmental cues. The interplay between jasmonic acid (JA)temperature in regulating chloroplast development remains poorly understood. Here, we demonstrate that methyl jasmonate (MeJA)warm temperature (28 °C) treatments additively impair chlorophyll accumulationchloroplast development in Arabidopsis thaliana cotyledons. We found that the thermosensor phytochrome B (phyB) suppresses, whereas the JA receptor CORONATINE INSENSITIVE 1 (COI1) promotes, MeJA-warm temperature-mediated chloroplast development. Moreover, phyB directly interacts with JASMONATE ZIM-DOMAIN 1 (JAZ1)JAZ3, preventing their JA-induced degradation,warm temperature attenuates this interaction. Strikingly, we reveal that transcription factors ELONGATED HYPOCOTYL 5 (HY5)MYC2 oppositely regulate cotyledon chloroplast development in response to MeJAwarm temperature by directlydifferentially modulating downstream transcriptional networks. Our study establishes a molecular framework in which phyB integrates JAwarm temperature signaling through the HY5-MYCs transcriptional regulatory network to fine-tune chloroplast development, highlighting a plant strategy for ecological adaptation.