The world we live in is increasingly multimodal, necessitating multiliteracy for learners to engage meaningfully with knowledge and skills. While verbal language is the primary medium for constructing, communicating and learning scientific knowledge and concepts, visuals can help learners to gain knowledge that they may not develop from verbal explanations alone. Verbal and visual semiotic modes have the potential to function in a complementary way, working together to strengthen meaning. This paper explores the dynamics of verbal-visual intersemiotic complementarity in the classroom practice of three junior secondary physical science teachers in the Oshana region of Namibia. It reports on a qualitative case study employing Systemic Functional Linguistics in the analysis of multimodal discourse. The results revealed how the content selections in the verbal and visual modes of the science teachers' practice are related in terms of the meanings and their relationships through all five intersemiotic sense relations. The results also showed that some teachers consistently employed fewer visuals. The results further highlighted low frequency of the use of intersemiotic antonymy (opposites) and hyponymy (hierarchy) by each teacher compared with intersemiotic repetition, synonymy (synonyms) and meronymy (whole-part relationships). It is recommended that Namibian science teacher training programmes incorporate intersemiotic complementarity in order to help teachers harness the full potential of this approach.