This study investigated the phenomena of group polarisation with particular attention to the differences between offline and online settings. Polarisation is a process that leads people to develop extreme ideologies. Three hundred and seven participants were recruited and randomly assigned to different experimental conditions, i.e. antisocial and prosocial polarisation, within groups of 6 people composed of four confederates, participating in discussions about a social dilemma under two different circumstances: face to face and online. The degree of polarisation was assessed considering the final decisions adopted by the participants, as well as the internal dynamics characterising their final attitudes, i.e. compliance versus conversion. Results showed that online groups appeared more susceptible to polarisation and their members reported a greater degree of conformism. In particular, within online environments, the risk of being polarised, both antisocially and prosocially, increased by around 12%. Furthermore, in an online setting, a greater degree of conversion emerged only when the members decided to adopt a pro-social behaviour, while a greater degree of compliance emerged whenever they decided to adopt antisocial behaviour.