This contribution focuses on the professional pedagogical relationship between trainee teachers and their mentors. This relationship has not yet been thoroughly investigated. The aim of the present study is to close this research gap by using a cross-sectional survey of 2583 trainee teachers and trained teachers to investigate the effects of different relationship dimensions (transparency, fairness, trust, and ambivalence) on the experienced demands and the associated stress symptoms during the teacher traineeship. The analysis was carried out using a latent moderated structural equation approach. The results show that demands caused by the core activities of teacher training as well as by colleagues has a significant positive relationship with the resulting stress symptoms. The more transparent, fair, trusting, and the less ambivalent the relationship with the mentor is experienced by the trainee teachers, the lower the resulting stress symptoms. The results are discussed regarding their relevance for the relationship work as a central training component of the second phase of teacher training.