Artificial intelligence (AI) has become increasingly embedded in EFL teaching, reshaping how teachers plan lessons, design materials, and provide feedback. However, while previous research has focused mainly on AI adoption, competence, and attitudes, less is known about how AI-mediated teaching influences teacher motivation in classroom practice. This qualitative study addresses this gap through the lens of Self-Determination Theory, examining autonomy, competence, and relatedness based on interviews, classroom observations, post-lesson reflections, and journals from eight Vietnamese EFL teachers using AI. The findings show that AI both supported and challenged teachers by expanding pedagogical options, enhancing competence through preparation and feedback, while also shaping decision-making and, at times, weakening teachers’ sense of personal voice and immediacy. The study argues that AI should be understood not only as an instructional tool but also as a motivational mediator that reshapes teachers’ agency, confidence, and connection in EFL classrooms.