To explain student biology teachers' intention to teach sustainable nutrition (SN) in classes using an extended model of the theory of planned behavior. Germany-wide online questionnaire study in November/December 2019. A total of 621 student biology teachers (mean age, 23.3 years; SD, 3.9 years; 77% female). Intention to teach SN. Theory of planned behavior variables (attitudes toward teaching, subjective norms, self-efficacy), intention to eat sustainably, attitudes toward SN, knowledge about SN, prior university education for sustainable development experiences. Descriptive statistics, bivariate Spearman correlations, and a path model are reported. The extended TPB model provided a moderate-to-high explanation of the intention to teach SN (R² = 0.50; P < 0.001). Respondents with a higher intention to eat sustainably in their daily lives reported more positive attitudes toward teaching, higher self-efficacy, and a higher teaching intention. Prior university education for sustainable development experiences also predicted the intention to teach. A higher level of knowledge about SN was only associated with higher self-efficacy. Lectures and seminars on SN in biology teacher training may foster student (biology) teachers' self-efficacy in teaching SN and ensure that they understand the importance of their subject-specific commitment, involvement, and attitudes in implementing SN in schools.