While the role of and consequences of being a bystander to face-to-face homophobic bullying has received some attention in the literature, to date, very little is known about the relationship between witnessing and perpetrating homophobic cyberbullying. It is also unknown how empathy and toxic online disinhibition might impact the association between witnessing and perpetrating homophobic cyberbullying. The present study focused on examining the longitudinal association between witnessing and perpetrating homophobic cyberbullying, and the potentially moderating role of empathy and toxic online disinhibition in this relationship. There were 1067 adolescents (age range from 12 through 15) included in this study, and they answered questionnaires about empathy, toxic online disinhibition, homophobic cyberbullying involvement (witnessing, perpetration), and face-to-face homophobic bullying involvement (witnessing, perpetration). One year later, adolescents also completed a questionnaire on the perpetration of homophobic cyberbullying. Results revealed an association between witnessing homophobic cyberbullying and homophobic cyberbullying perpetration, as measured over one year, while controlling for face-to-face homophobic bullying involvement. Further, empathy and toxic online disinhibition moderated the relationship.