Generative artificial intelligence (GenAI) is rapidly transforming education, including the ways students learn languages and experience motivation. While recent research has focused mainly on the affordances of GenAI, much less attention has been given to its potential risks for learners’ motivation and engagement. This paper addresses this gap by examining how GenAI may both enhance and undermine language learning motivation. We discuss how GenAI can affect the satisfaction or frustration of the three basic psychological needs – autonomy, competence, and relatedness – and how these effects may shift motivational orientations from intrinsic to extrinsic forms. We then explore how GenAI might reduce cognitive engagement, critical thinking, and assessment integrity when used uncritically. We also consider key methodological opportunities and challenges in researching GenAI-mediated language learning motivation. Based on this analysis, we identify key directions for future research agendas, including the balance between autonomy and dependence, sustaining motivation beyond novelty effects, rehumanizing digital learning, and redefining teachers’ roles in GenAI-rich contexts. We also call for theoretical renewal, ethical awareness, and methodological innovation in the study of GenAI-mediated motivation. A deeper understanding of these processes will allow future research and practice to harness GenAI’s potential responsibly, promoting motivation that is both sustainable and self-determined.