BackgroundCyberbullying is presently an alarming problem worldwide due to its impact on the emotions, behaviour and psychological well-being of not only the victims, but the bullies themselves and also bystanders.AimThis study aims to investigate bystanders’ behaviours in cyberbullying and associated factors.MethodsThis research is a cross-sectional study of 578 secondary school students in Bangkok, Thailand. Simple random sampling was used to select four secondary schools. Data were collected through online questionnaires which included four sections: (1) demographic data, (2) bystanders’ behaviour in cyberbullying (cyberbullying experience and attitude towards cyberbullying were included in this section), (3) parental attachment (Inventory of Parent Attachment-Revised), and (4) self-esteem (Rosenberg Self-Esteem Scale-Revised).ResultsIt was found that most occurrences of bystanders’ behaviour were: (a) willing to intervene or help victims (34.6%), (b) ignoring the cyberbullying (28%) or (c) partaking in cyberbullying (26.3%). Participants who either were victims of cyberbullying or knew of cyberbullying happening to friends or relatives and had high parental attachment would intervene more than those who never had such experiences. Participants who knew of cyberbullying happening to the general public had tendencies towards ignoring the cyberbullying. Participants who indicated they were bullies and had positive attitude towards joining in tended to partake in the cyberbullying more than those who were not.ConclusionFactors relating to the behaviours of bystanders in cyberbullying should be further explored to provide support in the discouragement of ignoring and averting participation in cyberbullying. Secure parental attachment is one important factor that should be instilled during childhood years.