It is seen that there are few studies on the role of the knowledgeable other when the literature on Lesson Study [LS] is examined. However, for LS to be better understood and applied in different cultures, the role of the knowledgeable other in LS process should be clarified. This study aimed to determine the role of the researcher as the knowledgeable other in the planning phase of LS in the Turkish context. The study used a qualitative research method of action research design. The study participants were two mathematics teachers selected by convenience sampling and working in a public middle school in one of Turkey's central districts of a big city in the Central Anatolian Region. The data were collected by video-camera recording and analyzed by content analysis. As a result of the study, it was determined that the knowledgeable other contributed to the teachers participating in the study within the framework of the following seven themes: teaching approach, mathematical process skills, planning, student recognition knowledge, material use, guidance, and questioning. At the end of the study, there was a discussion on these themes, and some suggestions were made.