Motivated teachers are more likely to exhibit enhanced interest, persistence, and confidence when engaged in professional development (PD); yet, facilitators often neglect teacher motivation when implementing PD. Self-determination theory suggests that social and environmental contextual factors influence the psychological need of relatedness which promotes motivation. Therefore, PD facilitators could enhance teacher motivation by emphasizing supportive factors when asking teachers to participate in novel learning activities. We drew on self-determination theory for this qualitative case study to examine eight secondary mathematics teachers' perceptions of relatedness when participating in lesson study (LS). Our results derive from semi-structured interviews, journal entries, and field notes. Analysis of data indicated that positive interpersonal dynamics and collective agency reflected contextual factors that increased perceived feelings of relatedness while negative interpersonal dynamics reduced perceptions of relatedness. Participants who experienced satisfaction of relatedness through supportive contextual factors credited them as increasing their learning during LS. Conversely, members who reported less relatedness satisfaction via negative interpersonal dynamics expressed various degrees of learning. Those perceiving interpersonal conflict reported diminished learning while participants experiencing substantive conflict reported increased learning. Understanding social and environmental factors shaping relatedness could guide PD facilitators who seek to promote teacher motivation during LS.