Teachers' technological pedagogical knowledge (TPK) is regarded as a critical prerequisite to effectively use technology during teaching across content domains. Previous findings on the availability of TPK, and its relation to other corresponding components of teachers' professional knowledge, however, were relatively mixed, probably because previous research most exclusively relied on indirect measures, such as self-reports of teacher knowledge. By applying a newly developed test-based instrument to assess teachers' conceptual and situational TPK, we investigated the cognitive conditions accounting for the availability of TPK. In Study 1 (N = 284), we demonstrated that the availability of TPK depended on the level of teacher expertise. In Study 2 (N = 120), we examined the connectedness of TPK to its corresponding knowledge components (i.e., technological knowledge, pedagogical knowledge). We showed that teachers’ TPK was only related to their pedagogical knowledge. The findings demonstrate that pedagogical expertise is a crucial prerequisite for TPK, which enables teachers to judge the potentials of technologies for varying teaching situations. •We present a theory-based and feasible approach of TPK assessment.•The TPK test comprised conceptual TPK and situational TPK.•Situational TPK was predicted by expertise and conceptual TPK.•PK, but not TK, was directly related to conceptual and situational TPK.