This study aims to describe the implementation of integrated Environmental Sustainability learning and critical reasoning skills among first-grade elementary school students. A qualitative descriptive method was employed, involving 31 students and their classroom teacher at a private elementary school in West Surabaya. Data collection techniques included observation and documentation during a seven-week learning process in the second semester of the 2024–2025 academic year. The learning design was aligned with the International Baccalaureate (IB) curriculum, particularly within the “Sharing the Planet” unit. Activities involved KWL-Charts, multimedia-based exploration of natural resources, field trips to model sustainable communities, and classroom-based environmental observations. Students engaged in problem identification, solution analysis, and decision-making regarding environmental issues, emphasizing 3R actions (Reduce, Reuse, Recycle) through project-based learning. Findings revealed that 87% of students demonstrated increased critical reasoning skills in proposing environmental solutions, and 92% showed improved understanding of sustainability concepts. The activities directly supported SDG 12 (Responsible Consumption and Production) and SDG 13 (Climate Action). This study concludes that integrating Environmental Sustainability learning with critical reasoning skills can effectively foster ecological awareness and empower young learners to actively contribute to global sustainable development goals (SDGs) from an early age.