Iconicity is a relationship of resemblance between the form and meaning of a sign. Compelling evidence from diverse areas of the cognitive sciences suggests that iconicity plays a pivotal role in the processing, memory, learning, and evolution of both spoken and signed language, indicating that iconicity is a general property of language. However, the language-specific aspect of iconicity, illustrated by the fact that the meanings of ideophones in an unfamiliar language are hard to guess (e.g., "shigeshige" 'staring at something' in Japanese), remains to be fully investigated. In the present study, native speakers of Japanese and English rated the iconicity and familiarity of Japanese ideophones (e.g., "gatagata" 'rattling', "butsubutsu" 'murmuring') and their English equivalents (e.g., "rattle," "murmur"). Two main findings emerged: (1) individuals generally perceived their native language as more iconic than their non-native language, replicating the previous findings in signed language, and (2) the familiarity of words in their native language boosted their perceived iconicity. These findings shed a light on the language-specific, subjective, and acquired nature of iconicity.