Organized extracurricular activities (EAs) constitute an important part of many young children's lives. However, the role of EAs in children's development during early childhood is poorly understood. The current study examined the associations between EA participation and a range of school readiness outcomes in a sample of 345 urban Chinese preschoolers. Using three waves of data collected on EA participation and applying growth mixture modeling, we discovered two distinct trajectory classes with respect to the breadth of EA participation as well as two classes for EA attendance intensity. With a series of covariates controlled for, children's greater initial levels of or rates of increase in EA breadth were related to better early math skills. Greater probabilities of belonging to the higher-intensity class were also associated with better early math skills in children. However, EA participation did not predict other aspects of children's school readiness, including Chinese reading, receptive vocabulary, expressive language, social skills, and problem behaviors. This study extends existing EA literature, which has primarily focused on school-aged populations in Western contexts, by demonstrating substantial individual variations in Chinese preschoolers' trajectories of EA participation. Our findings suggest that EAs seem to have little unique contribution to children's school readiness except for early math skills.