The aim of this cross-sectional study was to examine the individual (demographic characteristics, substance-using behaviors, emotional status, and negative outcome expectancy) and peer factors associated with ketamine use in Taiwanese adolescents. A total of 9,860 adolescents completed the self-report questionnaires without omission. Demographic characteristics, substance-using behaviors, emotional status, negative outcome expectancy, and peer factors were compared between the ketamine users and non-users using the Mann-Whitney U test and the Chi-square test, and then significant factors were further selected for stepwise logistic regression analysis to examine the associated factors of ketamine use. Sixty-nine (0.7%) participants reported having used ketamine in the past year. The results of logistic regression analysis indicated that ketamine users were more likely to use marijuana, smoke cigarettes, have peers using illicit drugs, and have a lower level of negative outcome expectancy regarding using ketamine than ketamine non-users. The associated factors found in this study should be taken into consideration when developing prevention and intervention programs for ketamine use in adolescents.