This study aims to analyse teachers' understanding of realistic mathematics education (RME) through a blended workshop on designing an RME-based learning trajectory. Data was collected via online pre-test and post-test, involving mathematics teachers of junior high schools in Banda Aceh, Indonesia. The tests were accompanied by interviews, chats on WhatsApp groups, and video recording. A descriptive analysis of the various sources of data indicated that the teachers made varying shifts in understanding the principles of RME. The teachers slightly altered their understanding of the intertwinement and level principles. However, their comprehension of the reality principle remained the same; they considered real problems as daily life problems only, even though some teachers had already written the meaning of real problems as problems that students can imagine. They also have already possessed a satisfactory understanding of the activity principle before the workshop. Moreover, nearly half of the teachers showed a good understanding of RME-based learning trajectories. The findings reported that teachers benefited from the blended workshop and obtained knowledge of the RME-based learning trajectory after participation. Finally, the implications for further research were discussed in this paper.