The purpose of this study is to determine the opinions of preservice elementary school mathematics teachers on the "Mathematical reasoning skill". 117 preservice elementary school mathematics teachers receiving education in the undergraduate program of elementary school mathematics teaching at the faculty of education at a university in the spring term of the 2014-2015 academic year constituted the study group of the research.The case study method among qualitative research approaches was used in the study, and the written opinion form consisting of 6 questions was used as a data collection tool. The content analysis was used when analyzing the data obtained from the study. According to the results of the study, it was determined that there are various opinions on what the mathematical reasoning skill means. While pre-service teachers generally say that they do take any lesson or seminar on mathematical reasoning skill, they think that mathematical reasoning skill should be included in the secondary school mathematics course curriculum and it is beneficial for students.