Invasive species threaten worldwide biodiversity. Success in facing this problem may be possible through the engagement of younger audiences. Here, a game was designed to teach children how to recognize invasive seaweed and to encourage them to participate in marine citizen science. The game was first tested in a pilot study, and then carried out in Salinas High School (Asturias, Spain). Game-based training consisted of an explanation of invasive species, followed by the 'Find invasive seaweed' game and was evaluated with tests before and after the activity. After training, students spent 3 months looking for invasive species. Four invasive species were detected in the study area. The results showed a positive impact on knowledge gains and significant differences in marine environmental awareness and conservation actions.