In a screening using our unique natural product library, the C-nucleoside antibiotic formycin A, which exerts strong anti-influenza virus activity, was rediscovered. Aiming to develop a new type of anti-influenza virus drug, we synthesized new derivatives of formycin and evaluated its anti-influenza virus activity. Structural modifications were focused on the base moiety and sugar portion, respectively, and >40 novel formycin derivatives were synthesized. Modification of the C-7 position of the pyrazolopyrimidine ring strongly contributed to improve the activity. In particular, excellent anti-influenza virus activity was observed in the NHMe (10), SMe (12), and SeMe (15) derivatives, in which heteroatoms were introduced. In addition, in the modification of the sugar moiety, the presence of a hydroxyl group and its stereochemistry greatly affected both the expression and intensity of the activity. Furthermore, the evaluation results of the 7-SEt derivative (29) and the 2'-modified derivative (59) suggested that structural modifications may reduce cytotoxicity.