Adaptive teacher support (i.e., support that is tailored to students' understanding) is considered crucial for students' learning. Previous research examined support adaptivity mostly in small-scale, observational studies without distinguishing between theory-based facets of support adaptivity. Yet, students' perceptions of different aspects of teachers' support adaptivity may be important for students' academic functioning. We investigated to what extent students perceive theory-based support adaptivity facets. We developed the Questionnaire on Teacher Support Adaptivity (QTSA) measuring students' perceptions of teacher support adaptivity and examined its reliability and validity in two samples of secondary school students (N[subscript study1] = 1682; N[subscript study2] = 1048). Overall, the QTSA provided reliable and valid scores of students' perceptions of teacher support adaptivity at the student and teacher level. One facet (non-adaptive support with much regulation upon high student understanding) should be used with some caution and needs further research. The QTSA scores converged with but also added to other constructs tapping into teachers' support adaptivity. Students' support adaptivity perceptions were distinguishable from students' perceptions of other teaching indicators (agency and autonomy support). The QTSA can provide both individual and class level feedback for practitioners and help researchers to gain more insight into possible differential effectiveness of different adaptivity facets.