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