Although posttraumatic stress disorder (PTSD) is highly comorbid with psychiatric disorders, little longitudinal research has been conducted to determine the potentially causal links. This study aimed to investigate the prevalence and comorbidity patterns of posttraumatic stress disorder symptoms and psychiatric symptoms among adolescents exposed to the 2008 Wenchuan earthquake in China and to examine the bidirectional prospective associations between PTSD symptoms and other psychiatric symptoms. A sample of 1573 adolescent survivors (45.8% male; mean age at initial survey was 15.0 years, SD = 1.3) completed a battery of standardized measures assessing symptoms of PTSD, depression, panic disorder, generalized anxiety disorder (GAD), separation anxiety disorder (SAD), social phobia, conduct disorder, and attention deficit hyperactivity disorder at 6 and 18 months post-earthquake. Among participants with PTSD symptoms, 91.9 and 94.0% had at least one comorbid psychiatric disorder at 6 and 18 months post-earthquake, respectively; however, among those without PTSD symptoms, 54.3 and 50.4% had at least one psychiatric disorder. PTSD symptoms were more likely to co-occur with subtypes of anxiety or depression symptoms than with behavior problems. Participants who were screened as having PTSD comorbid with depression or SAD at 6 months were less likely to recover from PTSD over time. Longitudinal analyses showed that symptoms of depression, GAD and SAD predicted increases in PTSD symptoms. In turn, PTSD symptoms predicted increases in GAD and panic disorder symptoms. Overall, our results support causal hypotheses of PTSD comorbidity. Specific multi-modal assessments and treatments targeting to both PTSD and its comorbidity disorders are warranted.