In addition to the arbitrary invention of words in human language, there are also a number of lexical items in languages generated from the symbolization of particular groups of sounds. The present study investigates the structural and semantic characteristics of “sound-symbolic words” (or onomatopoeic words) in Thai. Data were drawn from formal and informal written documents. The Royal Institute””s and Matichon””s dictionaries were used as representatives of formal documents, while 40 Japanese-to-Thai translated comics were used for the informal documents. Structurally, it was found that most sound-symbolic words are either monomorphemic or reduplicated. In relation to semantics, sound symbolization in the data shows semantic relations to human behaviors, activities, and surroundings.