This study aimed to compare the perception of short vs. long vowel contrasts in Japanese and Modern Standard Arabic (MSA) by four groups of listeners differing in their linguistic backgrounds: native Arabic (NA), native Japanese (NJ), non-native Japanese (NNJ) and Australian English (OZ) speakers. The NNJ and OZ groups shared the first language (L1), but differed in their familiarity with Japanese. In both Japanese and MSA, vowel length is phonemic. In contrast, vowel duration plays a more limited (although not insignificant) role in English. Of interest was the discrimination accuracy of NNJ listeners who learned Japanese as a second (L2) or foreign language in adulthood. As expected, the NA and NJ groups discriminated their L1 contrasts more accurately than all the other groups, but the NNJ listeners showed a significant shift in their perceptual behaviour and outperformed the OZ listeners who have no knowledge of Japanese in discriminating the Japanese vowel length contrasts. Furthermore, NNJ was the only group who did not differ in their discrimination accuracy for the Arabic and Japanese stimuli. Taken together, the results obtained in this study suggest that NNJ learned to discriminate Japanese vowel length contrasts to some extent, but the learning did not carry over cross-linguistically to the processing of vowel length contrasts in an unknown language.