Agmatine, a decarboxylated derivative of arginine, has emerged as a promising natural polyamine for promoting human health. Sake, a traditional Japanese rice wine, is known to contain elevated levels of agmatine. During multiple parallel fermentation (MPF), rice undergoes simultaneous saccharification by the filamentous fungus Aspergillus oryzae and ethanol fermentation by Saccharomyces cerevisiae. Notably, agmatine production is achieved in A. oryzae during solid-state cultivation, independent of S. cerevisiae. Despite the apparent absence of arginine decarboxylase (ADC) orthologs in its genome, the fungus demonstrates robust agmatine productivity, particularly in response to acidic stimuli. In the present study, AO090102000327 of A. oryzae strain RIB40 was identified as ADC, previously annotated as phosphatidylserine decarboxylase (PSD), encodes a 483-amino acid peptide. The protein exhibited pyruvoyl-dependent decarboxylase activity, favoring arginine over ornithine and showing no activity with phosphatidylserine.