Instant strong and responsive underwater adhesion is significant for many biomedical, industrial, and household applications. However, underwater adhesives simultaneously possessing these advantages are challenging to fabricate because strong and responsive adhesions are usually opposing properties. Herein, we have prepared bioinspired supramolecular polymeric adhesives containing complementary nucleobase moieties in nucleic acids and have demonstrated instantaneous underwater adhesion with an adhesion strength reaching as high as 1.5 MPa. Strong underwater adhesion is attributed to the high capability of the adhesives to substantially remove the interfacial water, which can be further modulated by temperature, light, and pH. Excellent underwater adhesion performances have been achieved on various substrates and under distinct aqueous conditions, showcasing the wide applications of the bioinspired nucleobase-containing adhesives developed here. This work opens up a new opportunity for fabricating underwater adhesives for diverse applications with the combined advantages of instant strong and responsive adhesion.