: Current research on self-efficacy beliefs of interdisciplinary science teaching indicates shortcomings in facing recent teaching challenges in secondary education and corresponding valid instruments. Thus, we designed the Self-Efficacy Beliefs of Interdisciplinary Science Teaching (SElf-ST) instrument based on a pedagogical content knowledge model for science teaching. We ensured the factorial validity of ten factors. To bring construct validity down to a round figure, we examined convergent and divergent validity in this paper. For answering the overall research question regarding arguments for the convergent and divergent validity of the interpretation of the SElf-ST instrument's test values (and related hypotheses), we analyzed data of pre-service, trainee, and in-service biology, chemistry, and physics teachers (n = 590) in a cross-sectional study. While the strong latent correlations of the ten SElf-ST factors with self-efficacy beliefs of interdisciplinary science teaching in primary education (r = 0.40 - 0.63, p < 0.01) indicate convergent validity, the rather weak correlations with self-efficacy beliefs of general teaching (r = 0.17 - 0.54, p < 0.01), self-rated content knowledge in science (r = 0.13 - 0.40, p < 0.01), and perceived stress (r = -0.13 - -0.19, p < 0.01) support different divergent validity intensities. Thus, assumed relations within the nomological net surrounding the self-efficacy beliefs of interdisciplinary science teaching construct were confirmed for secondary education. In sum, we shed light on a rarely explored aspect of construct validity in science education research regarding self-efficacy beliefs. Doing so, we gained strong arguments that the SElf-ST instrument's test values can serve as indicators of self-efficacy beliefs of interdisciplinary science teaching in secondary education.