Introduction The social media attitude of health science students might affect patients' opinion about the health profession and have negative impact on e‐professionalism. The aim of this study is to investigate the behaviour of Greek dental students on Facebook, focusing on potentially unprofessional posts and the online student‐patient relationship. Materials and methods Five hundred and twelve dental students in Greece answered an anonymous, 23‐item questionnaire including multiple‐choice questions about various topics, including Facebook profile settings and content shared by dental students, student‐patient relationship via Facebook; and students' perception about the impact of their online behaviour. Results 93.2% of responders had a Facebook profile and 80.5% admitted that their online attitude might affect patients' opinion about dental profession. However, 71.7% posted pictures from holidays, 41.5% from nightclubs, and 26.2% photographs wearing swimwear/underwear, while 12.8% expressed online political party predilection. One quarter of students in clinical years were Facebook friends with patients and 58% and 30% of them had online discussion about topics related or not to dentistry, respectively, while 6.8% of dental students had posted defamatory comments about the dental school, faculty members or academic staff on Facebook. Discussion In accordance with studies in other countries, most Greek dental students had a Facebook profile and, although the majority realised the impact of Facebook behaviour on e‐professionalism, a considerable percentage posted unprofessional content. Conclusion Dental students might fall into pitfalls when it comes to e‐professionalism. As social media are becoming an integral part of life, there is need to include e‐professionalism in dental education curriculum.