Phototropins (phot1 and phot2) are plant-specific blue light receptors for phototropism, chloroplast movement, leaf expansion, and stomatal opening. All these responses are thought to optimize photosynthesis by helping to capture light energy efficiently, reduce photodamage, and acquire CO₂. However, experimental evidence for the promotion of plant growth through phototropins is lacking. Here, we report dramatic phototropin-dependent effects on plant growth. When plants of Arabidopsis thaliana wild type, the phot1 and phot2 mutants, and the phot1 phot2 double mutant were grown under red light, no significant growth differences were observed. However, if a very low intensity of blue light (0.1 µmol m^-2 s^-1) was superimposed on red light, large increases in fresh weight up to threefold were found in those plants that carried functional PHOT1 genes. When the intensity of blue light was increased to 1 µmol m^-2 s^-1, the growth enhancement was also found in the phot1 single mutant, but not in the double mutant, indicating that phot2 mediated similar responses as phot1 with a lower sensitivity. The effects occurred under low photosynthetically active radiation in particular. The well-known physiological phototropin-mediated responses, including chloroplast movement, stomatal opening, and leaf expansion, in the different lines tested indicated an involvement of these responses in the blue light-induced growth enhancement. We conclude that phototropins promote plant growth by controlling and integrating a variety of responses that optimize photosynthetic performance under low photosynthetically active radiation in the natural environment.