Various researchers have reported that learners still have a limited conceptual knowledge of linear motion which may affect their learning achievement and motivation towards this subject matter. For this purpose, an investigation was conducted to find out the effect of the Project-Based Learning (PBL) approach with educational robotics on senior students' conceptual understanding of linear motion and learning motivation. This study used a quasi-experimental non-equivalent group design with a quantitative research approach. Participants were 78 senior students selected purposively from one school whose teachers have been trained on PBL by Rwanda Quality Basic Education for Human Capital Development project. A linear motion achievement test and a 5-point Likert motivation scale questionnaire were administered to the same groups of students. Descriptive and inferential statistics were used to analyse data quantitatively using Ms Excel and SPSS. Results have shown that students who were taught through PBL had more motivation toward learning linear motion and conceptualized better linear motion. The study recommends implementing Project-Based Learning (PBL) approaches, including educational robotics, and providing professional development for teachers to improve senior students' conceptual understand and motivation in linear motion.