Informal science learning has been proven to have effects on a variety of science learning outcomes. This study explored the direct and indirect influence of both prior and current informal science learning on science performance by using data from the Programme for International Student Assessment 2015. More than ten kinds of informal science learning activities were measured. Multi-level structural equation modelling was used to investigate the mediating role of science interest and self-efficacy in the relationship between informal science learning and science performance in the United Kingdom, Hong Kong, and South Korea. The results showed that prior informal science learning has a positive effect on current informal science learning and a direct influence on science performance. In addition, informal science learning significantly influenced science performance through science interest and self-efficacy. These results were consistent across three countries or regions with different cultures and science learning conditions even after controlling for background variables at student and school level.