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Photoreceptor phytochrome B (phyB) mediates a variety of light responses in plants. To further elucidate the molecular mechanisms of phyB‐regulated hypocotyl elongation, we performed firefly luciferase complementation imaging (LCI) screening for phyB‐interacting transcription factors (TFs). LCI assays showed that phyB possibly interacts with brassinazoleresistant 1 (BZR1), BZR2, AUXIN RESPONSE FACTOR 6 (ARF6), and several WRKY DNA‐binding TFs in a red light‐dependent manner. Furthermore, biochemical assays demonstrated that photoexcited phyB specifically interacts with non‐phosphorylated BZR1, the physiologically active form of a master TF in brassinosteroid (BR) signaling, and this interaction can be competitively interfered by phytochrome‐interacting factor 4. Furthermore, we showed that phyB can directly interact with the DNA‐binding domain of BZR1 and affect the enrichment of BZR1 on the chromatin of target genes. Moreover, our genetic evidence and RNA‐seq analysis demonstrated that phyB negatively regulates BR signaling. Together, we revealed that photoexcited phyB directly interacts with the TF BZR1 to repress BR signaling in Arabidopsis. [ABSTRACT FROM AUTHOR]