This theoretical position paper highlights how second language (L2) learning demotivation or decrease in motivation can be reconceptualized within the framework of Vygotskian Activity Theory (AT). Previous demotivation studies primarily focused on determining the demotivating factors. There has not been sufficient academic attention to the dynamic interaction between L2 learners and their potential factors for demotivation. In this paper, we attempted to apply AT in order to explain this intricate demotivation process, particularly focusing on L2learners’ subjective perceptions of their learning environment. From the perspective of AT, L2 learners’ learning-related activities are seen as activities composed of subject, object, mediational tool, community, rule, and division of labor. Describing a learner’s L2 learning experience in Lantolf and Genung’s (2002) study, we argue that the tensions emerging between the elements in the activity system result in demotivation. Future directions and the issue of commensurability of AT with other theories are discussed.