Outdoor education (OE) represents an established approach to connecting learning with real-world contexts and supporting sustainability-related competencies. The aim of this study was to examine how Slovenian lower secondary natural science and biology teachers perceive and implement OE, as well as which factors influence its use in school practice. In Slovenia, natural science is taught as an integrated subject in grades 6–7, while biology is taught as a separate subject from grade 8 onwards; this study therefore included teachers from both instructional contexts. Data were collected using an online questionnaire completed by 108 teachers and analysed using descriptive statistics, correlations analyses, and Principal Component Analysis (PCA). The results indicate that teachers generally perceive OE as highly beneficial for students, particularly regarding engagement, experiential learning, and environmental awareness, but report obstacles such as limited time, curriculum overload, and large class sizes. These findings provide empirical insight into the gap between curricular intension and classroom practice and may inform policy measures and teacher education programmes aimed at strengthening the systematic integration of OE in Slovenian science teaching.