Background: We examined the effects of the Galaxy Rescuers game, a collaborative board game on schoolchildren's bullying intervention. Methods: We conducted a group-randomized controlled trial. We recruited 328 fifth graders at an elementary school in northern Taiwan. The study took place in fall 2015 over a 7-week period. We used the generalized estimation equation (GEE) to evaluate the intervention effects on students' scores on the outcome measures. Results: At posttest, the change in bullying knowledge was statistically significant for the game-only group and the game-with-debriefing group. Students in the game-with-debriefing group also showed an increase in empathy and a decrease in bullying attitude. At the follow-up test, knowledge increase remained significant for both the game-only group and the game-with-debriefing group. Game satisfaction survey indicated that 66.8% of the players said that the game was enjoyable. Conclusions: The Galaxy Rescuers game is effective in changing players' bullying knowledge, attitude, and empathy. This game is an entertaining tool for promoting awareness of bullying and encouraging defending atmosphere among children.