Sugars function as signal molecules that regulate gene expression, growth, and development in plants, animals, and yeasts. As a contribution to understanding the molecular mechanisms of sugar responses, we identified and characterized an Arabidopsis thaliana BTB domain-containing protein of the H subfamily 1 (designated AtBTH1). The relative expression level of AtBTH1 was enhanced by exogenous sugar treatment. To determine the function of AtBTH1 in sugar response, we identified two T-DNA insertion mutants, atbth1-1 and atbth1-2, which displayed an insensitive response to sugar-mediated inhibition of seed germination. The relatively early seedling growth of the atbth1-1 and atbth1-2 mutants was promoted by exogenous sugar treatment. Light-grown seedlings of the atbth1-1 and atbth1-2 mutants developed long roots when grown on medium containing a high sugar concentration. Dark-grown atbth1-1 and atbth1-2 seedlings showed sugar-insensitive hypocotyl elongation and development. These findings revealed that expression changes caused by mutation of AtBTH1 altered sugar-responsive seed germination and seedling growth. Taken together, the results suggest that AtBTH1 plays an important role in sugar responses in Arabidopsis.