Research suggests that supportive teacher-student relationships are a prerequisite for student development. Developing such relationships requires teachers to observe, interpret, and reflect on teacher-student interactions and on teachers' relational competence in practice. Although teacher-student relationships are especially challenging with students with attention deficit hyperactivity disorder (ADHD), few studies have examined how these relationships develop. In this qualitative study, teachers from two Swedish elementary schools comprising one intervention (n = 33) and one control group (n = 20) completed pre- and post-tests in which they analysed videos of teacher-student interactions. We explored how the teachers understood relational competence in relation to students with ADHD before and after the video-based intervention, which included a presentation on the concept of relational competence and a model for analysing such competence. Our findings suggest that the intervention promoted teacher development regarding relational competence. Compared with the pre-test period, participants (i) used more nuanced relational language, (ii) substantiated their claims with concrete cues regarding interpersonal communication, and (iii) adopted both teacher and student perspectives regarding the relationship. Finally, new understanding regarding relational competence was combined with knowledge regarding the importance of teacher sensitivity and responsiveness when working with students with ADHD.