Plants coordinate their growth and developmental programs with various endogenous signals and environmental challenges such as seasonal and diurnal temperature fluctuations. The bHLH transcription factor PIF4 plays critical roles in thermoresponsive hypocotyl growth in Arabidopsis, and the evening complex component ELF3 negatively regulates PIF4's activity for downstream gene expression and hypocotyl elongation at elevated temperature. However, how warm temperature signal is transmitted to ELF3 is not known. Here, we report the identification of two B-Box protein BBX18/BBX23 as new regulators of thermomorphogenesis in Arabidopsis. Mutations of BBX18/BBX23 confer reduced thermoresponsive hypocotyl elongation. Overexpression of BBX18 enhances the sensitivity of hypocotyl growth to elevated temperature, which is dependent on the function of PIF4 and RING E3 ligase COP1, respectively. Both BBX18 and BBX23 interact with ELF3 or COP1, relegating the protein abundance of ELF3 at warm temperature. Further, the expression of multiple thermoresponsive genes is impaired in both the PIF4 single mutant and BBX18/BBX23 double mutant. In addition, both the transcription and protein levels of BBX18/BBX23 are up-regulated by elevated ambient temperature. Thus, our findings reveal the important roles of B-Box proteins in plant thermomorphogenesis, and build a new connection from warm temperature information to ELF3 and its downstream signaling components. Overall design: Totally 18 samples with three genotypes and two treatments,both of which are in triplicates.