Botulism is a rare neuromuscular disorder caused by neurotoxins produced by Clostridium botulinum. The diagnosis of infant botulism may be obscured or delayed, as its presentation is similar to that of infantile neuromuscular disorders. We report the first Korean case of infant botulism in an acute progressive floppy infant with poor sucking and a weak cry. No abnormalities were found in all blood, cerebrospinal fluid, genetic test, nerve conduction study, and imaging studies. A stool-toxin test was finally performed under suspicion of infant botulism, and the result was positive. The patient was immediately treated with heptavalent botulism antitoxin. Follow-up after 3 months showed normal development with a complete resolution of all symptoms. Therefore, clinical suspicion of infant botulism, which is a treatable infantile neuromuscular disease, is essential for early diagnosis and prompt treatment in the differential diagnosis of a floppy infant.