Transphobic bullying is prevalent in schools around the world. The present study examined the bidirectional relationships among transphobic attitudes, perpetration of transphobic bullying, and witnessing transphobic bullying. The study also included the validation of an instrument to measure transphobic bullying and the prevalence rates of transphobic attitudes and behaviors. The sample was composed of 567 adolescents (38.8% girls; aged 12-17; M = 13.96; SD = 1.18). Participants completed measures of transphobic attitudes, perpetration of transphobic bullying and witnessing transphobic bullying at time 1 (T1) and time 2 (T2) during a 1-year interval. Results showed that transphobic attitudes predicted perpetration of transphobic bullying 1 year later, and that perpetration of transphobic bullying predicted witnessing transphobic bullying 1 year later. As expected, boys scored higher on the measures of perpetration of transphobic bullying and transphobic attitudes at both T1 and T2. These results indicate the importance of working to eliminate transphobic attitudes to reduce bullying behavior.