The purpose of this study was to describe in-service health education teachers' experiences teaching sexuality education to students in secondary schools in Japan. The study was informed by the theory of teacher development. The methodology used in this study was qualitative and descriptive and adopted a case study design. Participants were five (four male and one female) in-service health education teachers with experience in sexuality education. Constant comparative method was used to analyse and interpret the data. Thematic analysis resulted in three themes: (a) the need to teach comprehensive sexuality education, (b) the pedagogical shift from face to face to online education, and (c) the importance of professional development in sexuality education. Experiences depicted by each theme indicate that Japanese health education teachers should be trained to support students from diverse academic, social, and cultural backgrounds in their sexuality education classes, enhancing both teaching and learning.