The study investigated preschool teachers' pedagogical content knowledge (PCK) in mathematics. The construct of PCK for teaching mathematics in preschool involves three components: (1) noticing mathematical situations in which children engage; (2) interpreting the nature of children's math activity; and (3) enhancing children's mathematical thinking and understanding. The research participants were 30 preschool teachers from a large city in South Korea. Teachers identified mathematical situations in a children's play scenario, interpreted the nature of the mathematical situations, and identified how children's mathematical thinking in the reported situation could be enhanced. The teachers' responses were quantitatively scored. The results indicate that participants possessed higher levels of PCK about number sense, measurement, and classification than for patterning, operations, shapes, and spatial relationships. Teachers need knowledge to interpret mathematical situations in order to identify ways to enhance children's mathematical thinking. Teachers with more teaching experience were more likely to have higher scores on the overall PCK measure. Teachers require greater knowledge to recognize specific mathematical concepts in use in children's play in order to be able to extend and enhance preschool children's mathematical thinking.