Although self‐assessing one's knowledge is an integral part of a teacher's professional development, little is known about the accuracy of teacher candidates' self‐assessments. In particular, not much attention has been paid to their self‐assessment accuracy assessed on an individual level. Using measures from research on metacognition, we investigated the accuracy and bias of 513 mathematics teacher candidates who were tested in and self‐assessed their professional knowledge in the three core domains of mathematical content knowledge, pedagogical content knowledge and pedagogical knowledge. In addition, we examined the consistency of self‐assessment accuracy across the three knowledge domains and its relation to personal characteristics such as cognitive ability, personality traits and occupational self‐efficacy. Results showed that in all three domains of professional knowledge, most teacher candidates were either over‐ or underconfident in their knowledge and unaware of their strengths and weaknesses. Cognitive ability reduced the risk of being overconfident, whereas self‐efficacy was predictive of overconfidence. The Big Five personality traits were not systematically connected to self‐assessment accuracy, and variance in self‐assessment accuracy is still to be explained by further research. We address shortcomings of measures that are commonly used in self‐assessment research and advocate for the use of metacognitive methods in future studies that focus on teacher self‐assessments.