This study examined the effectiveness of an Eco-Friendly Education Program (EDEP) on the sustainability behaviors and environmental awareness of preschool children aged 60–72 months. Employing a quasi-experimental pretest–posttest control group design, the research was conducted in a public preschool in Sakarya, Türkiye, with an experimental group (n = 20) and a control group (n = 24). The EDEP comprised 16 integrated activity sets encompassing 49 individual activities organized around six thematic areas—plant life cycles, recycling, pollution prevention, energy conservation, climate change awareness, and sustainable nutrition—delivered twice weekly over eight weeks with a cumulative contact time of approximately 12 hours. The control group followed the standard Ministry of National Education (MoNE) preschool curriculum without supplementary environmental content. Data were collected using two validated instruments: the Sustainable Behaviour Assessment Scale (SBAS; Özkan et al., 2019; 20 items, 5-point Likert, two sub-dimensions) and the Environmental Awareness and Attitude Scale for Preschool Children (EAASPC; Büyüktaşkapu Soydan & Öztürk Samur, 2017; 26 items, two sub-dimensions). As Shapiro–Wilk tests indicated non-normal distributions, Mann–Whitney U and Wilcoxon signed-rank tests were employed. Results revealed statistically significant improvements in the experimental group across all sub-dimensions of both scales (p < .001), with all 20 children demonstrating positive change on multiple measures. The control group showed mixed results: significant gains on some sub-dimensions but an unexpected decline in environmental awareness scores. Post-test between-group comparisons confirmed that the experimental group significantly outperformed the control group on all sub-dimensions (p < .001), with large effect sizes (r = .75–.85). These findings provide robust evidence that structured, multi-component environmental education during early childhood can meaningfully enhance both sustainability behaviors and environmental awareness, supporting the broader integration of such programs into preschool curricula.