The Internet is gaining relevance as a platform where extremist organizations seek to recruit new members. For this preregistered study, we developed and tested a novel online game, Radicalise, which aims to combat the effectiveness of online recruitment strategies used by extremist organizations, based on the principles of active psychological inoculation. The game "inoculates" players by exposing them to severely weakened doses of the key techniques and methods used to recruit and radicalize individuals via social media platforms: identifying vulnerable individuals, gaining their trust, isolating them from their community and pressuring them into committing a criminal act in the name of the extremist organization. To test the game's effectiveness, we conducted a preregistered 2 × 2 mixed (pre-post) randomized controlled experiment (n = 291) with two outcome measures. The first measured participants' ability and confidence in assessing the manipulativeness of fictitious WhatsApp messages making use of an extremist manipulation technique before and after playing. The second measured participants' ability to identify what factors make an individual vulnerable to extremist recruitment using 10 profile vignettes, also before and after playing. We find that playing Radicalise significantly improves participants' ability and confidence in spotting manipulative messages and the characteristics associated with vulnerability.