Given the importance of fostering students' interest as a goal of physics education in meeting international science standards, empirical support for the theoretical description of the interest construct is essential. Empirical studies require the use of psychometrically sound measurement instruments. This study developed an instrument to measure students' interest in particle physics (IPPI). Drawing from previous research, we defined interest in particle physics, identified corresponding behaviours, and proposed a hierarchy of students' levels of interest in particle physics. Then, we developed the IPPI, using rating scale items that assessed the latent trait developed from our theory regarding the degree of interest in particle physics. We tested the IPPI in student think-aloud interviews and validated it in a field test on a sample comprising 99 German-speaking grade 9 students. A Rasch analysis provided evidence supporting the content, construct, statistical, and fit validity of the IPPI. We revised the hypothesised hierarchy of students' levels of interest in particle physics based on the item hierarchy revealed by the Rasch analysis. We associated each level with different contexts, such as socio-scientific issues. Knowing about these levels of interest in particle physics can help educators design their learning activities better and foster their students' interest.