Second language acquisition theory has traditionally focused on the cognitive and psycholinguistic processes involved in additional language (L2) learning. In addition, research on learner psychology has primarily centered on learners' cognitive abilities (e.g., aptitude and working memory) and internal traits or states (e.g., dispositions, motivations, and affect). Language learning behavior, however, has remained largely neglected and under-theorized. To address this gap, this paper proposes the proactive language learning theory, which delineates the agentic and strategic behaviors that learners employ to learn an additional language. These behaviors include input-seeking behavior, interaction-seeking behavior, information-seeking behavior, and feedback-seeking behavior. This paper presents theoretical arguments supporting the proposal, describes the four behavioral dimensions of the theory, and outlines general hypotheses concerning the contextual and learner-related antecedents of these behaviors and their effects on L2 outcomes. Finally, the potential implications of this theory for advancing our understanding of L2 learning and instruction are discussed.