The aim of this study is to understand how /r/ emerged and developed in Proto-Japanese and how the conditions of its emergence shed light on its present phonological behavior. The paper first offers a review of the phonetic, phonological, and morpho-phonological characteristics of /r/ in Japanese through examination of a large array of empirical evidence. The picture that emerges is that of an unmarked, phonologically empty segment, confirming a number of previous studies, in particular that by Mester and Itoˆ (Language 65:258-293, 1989). I argue that /r/ primarily developed in Japanese as a default epenthetic consonant in the intervocalic position within the morphological domain of a stem and its affixes, through an 'emergence of the unmarked' mechanism before becoming a fully contrastive phoneme later on by virtue of a phonologization process. A formal account of this proposal within the framework of Optimality Theory is offered. It is shown that the phonological content of /r/ is acquired due to the application of well-formedness constraints (ONSET, ALIGN, MAXIO, DEPIO) as well as that of two sets of markedness constraints (FEATURAL AGREEMENT and HARMONY SCALE), which ensure that the null input is mapped to the least marked output in terms of phonological features.