English as a foreign language (EFL) teachers differ in their autonomy-supportive and controlling interpersonal behavioral styles when motivating learners. Drawing upon the self-determination theory (SDT) and the model of interpersonal teacher behavior (MITB), this study aimed to investigate the impact of teachers' interpersonal behaviors, according to eight behavioral dimensions: leadership, helpful/friendly, understanding, student responsibility/freedom, uncertain, dissatisfied, admonishing, and strict. 1,209 Iranian EFL learners were requested to assess their teachers' interpersonal behavior via the Questionnaire on Teacher Interaction (QTI) as well as their degree and type of motivation (autonomous and controlled) through the Academic Self-Regulation Questionnaire (SDQ-A). The results of the partial least squares structural equation modelling revealed that teachers' controlling behaviours of leadership and strictness significantly enhanced EFL learners' controlled motivation, while their autonomy-supportive behaviours of helpfulness/friendliness and understanding positively influenced learners' autonomous motivation. Teachers' autonomy-suppressive behaviour of uncertainty negatively influenced learners' autonomous motivation. This study's findings encourage stakeholders in the educational sector to consider EFL teachers' interpersonal behaviours and the influence of these behaviors on learning motivation.