This research aimed to investigate the relationship between mathematical epistemological beliefs and the mathematics motivation of high school students. The survey model was employed in the research. Four hundred twenty-four high school students constitute the study group from public high school. Data were collected through the "Mathematics-Oriented Epistemological Belief Scale" and the "Mathematics Motivation Scale." The Mann-Whitney U and Kruskal-Wallis H tests were used to analyze the data. The findings showed a significant positive relationship at a moderate level between the belief that learning depends on effort with motivation, a weak and negative significant relationship between the belief that learning depends on ability with motivation and a negative and insignificant relationship between the belief that there is only one truth with motivation. The mathematical epistemological beliefs and mathematical motivations of high school students differ statistically in terms of gender, mother's education level, and daily studying time. However, they do not differ statistically regarding out-of-school support, technology, internet use, class level, father's education level, or perceived income level.