Mutations in the hepatocyte nuclear factor (HNF)-1α gene have been linked to subtype 3 of maturity-onset diabetes of the young (MODY), a disease characterized by a primary defect in insulin secretion. Here we show that the human GLUT2 gene is closely regulated by HNF-1α via sequences downstream of the transcriptional start site by interaction with transcriptional co-activator p300. The promoter region of the human GLUT2 gene was subcloned into luciferase expression plasmids that were transfected together with HNF-1α expression plasmid into a pancreatic β-cell line, HIT-T15, to evaluate transcriptional activities. HNF-1α enhanced human GLUT2 promoter activity sixfold. Site-direct mutagenesis and footprint analyses showed that the HNF-1α binding site (+200 to +218) is critical in human GLUT2 gene expression. Furthermore, mammalian two-hybrid and immunoprecipitation studies revealed the transactivation domain of HNF-1α (amino acids 391–540) to interact with both the NH₂-terminal region (amino acids 180–662) and the COOH-terminal region (amino acids 1,818–2,079) of p300. These findings demonstrated that HNF-1α binds to the 5′-untranslated region of GLUT2 and that p300 acts as a transcriptional co-activator for HNF-1α. In addition, these results provided new insight into the regulatory function of HNF-1α by suggesting a molecular basis for human GLUT2 gene expression.