Plastic marine debris (PMD) polluting marine habitats is a pressing anthropogenic global environmental problem and its reduction requires the commitment of government and industry, and the collaboration of the public. Environmental citizen science projects (CSPs) have flourished and are widely regarded as having positive effects on participants’ knowledge, perception and behavior. This study examines how participation in a CSP on PMD, which included a scientific sampling of PMD on local beaches, affected Chilean schoolchildren’s (9–18 years) problem perception and personal involvement, including self-reported behavior. A pretest-posttest design was used, with an experimental group (CSP participants, n = 494) and a control group (n = 318). Educational and behavioral effects of the intervention were assessed using items based on the norm activation model. Both groups showed high initial problem perception and involvement regarding PMD. A mixed model multivariate analysis of variance revealed that engagement in the CSP did not result in significant changes of almost all dependent variables, except for a small positive effect on ascription of harm. Age substantially affected the outcomes and was included as a covariate. The findings suggest that pro-environmental behavior change cannot be expected from participation in environmental CSPs alone; it requires the incorporation of auxiliary educational activities in the project design specifically conceptualized for targeting this learning objective.