Contextual knowledge is crucial for teachers in integrating technology into teaching. Drawing on the Ecological Systems perspective, this study examined how a self-constructed contextual knowledge cluster correlates with technological pedagogical content knowledge (TPACK). Factor analysis and canonical correlation analysis of a survey on 136 Vietnamese EFL teachers indicated that contextual knowledge is a TPACK subdomain, and that teachers’ ability to address contextual influences could mitigate the impacts of personal attitudes, institutional conditions, and national policy.