Although many studies have elucidated the mechanisms by which different wavelengths of light (blue, red, far-red or ultraviolet-B [UV-B]) regulate plant development, whether and how green light regulates plant development remains largely unknown. Previous studies reported that green light participates in regulating growth and development in land plants, but these studies have reported conflicting results, likely due to technical problems. For example, commercial green LED light sources emit a little blue or red light. Here, using a pure green light source, we determined that unlike blue, red, far-red or UV-B light, which inhibit hypocotyl elongation, green light promotes hypocotyl elongation in Arabidopsis thaliana and several other plants during first the 2–3 days after planting. Phytochromes, cryptochromes, and other known photoreceptors do not mediate green light-promoted hypocotyl elongation, but the brassinosteroid signaling pathway is involved in this process. Green light promotes the DNA binding activity of BRI1-EMS-SUPPRESSOR 1 (BES1), a master transcription factor of the brassinosteroid pathway, thus regulating gene transcription to promote hypocotyl elongation. Our results indicate that pure green light promotes elongation via brassinosteroid signaling and acts as a shade signal to enable plants to adapt their development to a green-light-dominant environment under a canopy.