As global environmental challenges intensify, there is an increasing need to equip younger generations with the knowledge, values, and sense of responsibility necessary for a sustainable future. This study explores how environmental citizenship education (ECE), implemented through a nature-based learning program within a Nature and Science School (NSS) in Türkiye, was experienced and interpreted by primary school pupils in relation to their development of understanding of sustainability and environmental citizenship. NSSs, integrated into the formal education system by the Turkish Ministry of National Education, offer inquiry-driven and experiential learning in natural settings. The study took place in Talat Göktepe Grove, a biodiverse site including forest and marine ecosystems, where a four-month ECE program was conducted. A holistic single-case study design was employed, drawing on pupil diaries and semi-structured interviews. A total of 88 pupils engaged in structured outdoor activities addressing biodiversity, sustainability, and the climate crisis. Initially, pupils described environmental citizenship through individual actions. Over time, their perspectives expanded to include civic participation, environmental rights, and collective responsibility. Their reflections also revealed a more nuanced understanding of sustainability, encompassing concepts such as ecosystem balance, renewable energy, and environmental justice. The study provides insight into how nature-based education may support meaning-making around environmental citizenship and sustainability in early education.