Although the international interest in teaching probability has allowed probability to be treated as a separate learning area in Turkey, its intensity has been reduced and mitigated in the middle school mathematics program. Despite this, the addition of statistics and probability courses for middle school mathematics teacher candidates during their undergraduate education shows the need for them to be trained in these subjects’ teaching. However, it is claimed that the probability knowledge that teacher candidates receive in their undergraduate years is not sufficient; they cannot learn probability with different approaches, and therefore do not have the necessary competence to teach probability. In this case, it is necessary to examine the probability knowledge in terms of common content knowledge (CCK), advanced content knowledge (ACK), and specialized content knowledge (SCK) required to teach the probability of mathematics teacher candidates. In this study, probability content knowledge (CCK, ACK, and SCK) of mathematics teacher candidates was examined in the context of different meanings of probability (classical, frequentist, and subjective). According to the general results obtained by applying the probability content knowledge test, which was adapted to Turkish to 98 teacher candidates, it was found that the content knowledge in which participants were most successful was CCK and had a sufficient level of understanding for the classical meaning of probability. However, it was found that there are deficiencies in teacher candidates for ACK and SCK, and their understanding of the frequentist and subjective approach of probability is insufficient. It was recommended to improve and expand the scope of statistics and probability courses given during university education. In parallel, restructuring middle school mathematics programs could be applied to emphasize classical and other meanings of probability.