Language learning is context-dependent and requires learners to employ different sets of language skills to fulfill various tasks. Yet standardized English as a foreign language assessments tend to conceptualize English proficiency as a unidimensional construct. In order to distinguish English proficiency as separate context-driven constructs, I adopted a register-based approach to investigate academic English proficiency (i.e., specific set of language skills that support academic literacy) and general English proficiency (i.e., wide range of language skills undifferentiated by context that are measured by traditional assessments) as separate predictors of overall essay quality. In the study, students completed a general English proficiency assessment and an academic language proficiency assessment, and essays were coded for academic writing features at the lexical, syntactic, and discourse levels. Beyond the contribution of academic writing features and general English proficiency, academic English proficiency emerged as a significant contributor to essay quality. Findings suggest that academic English proficiency scores more precisely identified a subset of academic language skills that is relevant to essay writing. The article concludes by discussing implications for strategic writing instruction that articulates the key expectations of academic writing used in and beyond school contexts.