Building on the extensive work conceptualizing, developing, and validating foreign language aptitude for successful second language acquisition (e.g., Skehan, 2015, in Applied Linguistics), the current project expounded the cognitive abilities relevant and instrumental to adult Japanese classroom learners’ pronunciation attainment of English /ɹ/. The speech production of 50 Japanese participants was elicited, acoustically analyzed, and linked to different aspects of their aptitude profiles (phonemic coding, associative memory, and sequence recognition). Whereas those with higher phonemic coding demonstrated better performance in a relatively easy dimension of English /ɹ/ pronunciation (lower F2 for tongue retraction), those with greater associative memory demonstrated more advanced performance in the relatively difficult dimensions of English /ɹ/ pronunciation (longer transition duration for phonemic length; lower F3 for labial/alveolar/pharyngeal constrictions). The role of incidental learning aptitude (i.e., sequence recognition) remained unclear. The findings here indicate that explicit aptitude related to phonological analysis and memory may play a key role in predicting the incidence of advanced second language segmental proficiency attainment in classroom settings.