Onomatopoeic words in Japanese constitute an independent lexical-semantic system consisting of five groups. One of the most fundamental features of onomatopoeic words in Japanese is the regular, systematic connection between phonetic form and meaning. This article examines the lexico-semantic structural features of onomatopoeic lexicon in Japanese, focusing on the categories of giongo (擬音語) and gitaigo (擬態語), analyzing their classification, phonosemantic isomorphism, and grammatical usage through concrete examples. The article demonstrates that Japanese possesses a uniquely developed onomatopoeic system compared to other world languages.