Brassinosteroid is necessary for sugar promotion of Arabidopsis hypocotyl elongation in darkness, and sugar positively regulates BRASSINAZOLE RESISTANT1 (BZR1) at both transcription and protein levels. Sugar has the ability to induce Arabidopsis hypocotyl elongation in the dark, but the detailed mechanisms remain not well understood. Here, we report that the steroidal phytohormone brassinosteroid (BR) is involved in sugar promotion of hypocotyl elongation in the dark. Sugar-induced hypocotyl elongation was significantly repressed in the BR-deficient mutant det2-1, BR-insensitive mutant bri1-5, and wild-type plants (Col-0), but not in the BR-hypersensitive mutants bzr1-1D and bes1-D treated with the BR biosynthetic inhibitor brassinazole (BRZ). Sugar also up-regulated the expression of genes that are related to cell elongation in a BR-dependent manner, and this effect was more remarkable in bzr1-1D and bes1-D than in their corresponding wild types in the presence of BRZ, suggesting an important role of BZR1 and bri1-ems-suppressor 1 (BES1) in this process. Sugar treatment seems to have little effect on BR biosynthesis, but enhances the expression of BZR1 and BES1, two transcription factors in BR signaling, in the dark. Furthermore, sugar treatment maintains higher BZR1 protein levels in plants grown in the dark. Collectively, our results indicate that BR is required for sugar promotion of hypocotyl elongation in darkness in Arabidopsis.