Despite the substantial amount of literature concerning adolescent bystanders of online hate and cyberbullying, relatively little attention has been devoted to studying the same issue in adults. Similarly, the determinants of the effectiveness of different messages to support the victims or counter hate have also been understudied. The existing pieces of empirical research on these topics remained scattered and no systematic review was performed to check if there are any patterns with regard to determinants and consequences of adult bystanders intervening against hate online. To fill these gaps, we performed a literature review in accordance with the guidelines of the Cochrane Collaboration Handbook for Systematic Reviews. The results of the literature search and analysis yielded three important findings. First, personal and contextual factors determining bystander action in adults largely overlap with the factors identified in adolescent populations: empathy, prior victimisation, feelings of responsibility, severity, social norms, relationship with the victim and number of bystanders. Second, personal factors promoting bystander action seem to be interconnected via empathy and social norms, both of which can be facilitated through psycho-education. Third, there is a critical lack of studies on the effectiveness of different bystander interventions.