The identification of new adjuvants is a critical need in vaccinology. In this work, it is demonstrated that bis-(3′,5′)-cyclic dimeric guanosine monophosphate (cdiGMP) exhibits potent adjuvant properties. Subcutaneous co-administration of cdiGMP with β-galactosidase (β-Gal) to mice resulted in the elicitation of significantly higher antigen-specific serum IgG titres than in animals receiving β-Gal alone. Strong cellular immune responses, which were characterized by a balanced Th1/Th2 pattern, were also observed in response to the β-Gal protein and a peptide encompassing its MHC class I-restricted epitope in immunized animals. These results suggest that cdiGMP represents a promising adjuvant for vaccine development.