The aim of this study was to reveal pre-service elementary mathematics teacher' knowledge about the concept of polygon. The study utilized the case study design. The sample of the research consisted of 23 fourth grade students studying in the primary school mathematics teaching department. Purposeful sampling method was used in selecting the participants of the study. The data of the research was collected using interview method. The interview questions consisted of four questions aiming to reveal whether the students knew the defining features of the polygon concept. Interview questions were analyzed with the content analysis. As a result of the analyses, in addition to the correct answers given by the students to the questions, their incorrect answers were also observed. All students correctly answered the question about "Polygon is a closed shape", one of the defining characteristics of the polygon concept. They partially correctly answered the questions about the defining characteristics of the polygon concept: "Line segments do not intersect except for their endpoints", "The sides of a polygon consist of line segments", "Line segments with common endpoints cannot be linear". In order to prevent wrong answers, methods that will help the student adopt the definition of the concept in the best way should be used.