The current study aims to investigate the interplay between teacher and student emotions in mathematics and the triggers of student emotions from the perspectives of their mathematics teachers. Mixed-methods research was employed. In the quantitative phase, 214 middle school mathematics teachers and their seventh and eighth graders (5475 students total) participated in the study to explore the relationship between teacher and student emotions in mathematics. Afterward, 14 mathematics teachers were selected to participate in the qualitative phase according to the variation in class mean values of the student achievement emotion scale. Data were collected through questionnaires (Achievement Emotions Questionnaire-Mathematics, Teacher Emotions Scale) and semi-structured interviews. Multilevel structural equation modeling revealed no significant correlation between teacher enjoyment, anger, anxiety, and student emotions. Thematic analysis unraveled four themes in explaining the factors affecting students' emotions in mathematics: types of emotions in mathematics learning, sources of emotions, consequences of emotions, and strategies to regulate students' emotions in mathematics. This study stands as the pioneering one in emotional transmission between teachers and students in Turkiye.