Teachers' adaptivity of constructivist and direct-transmissive beliefs may be beneficial for students' learning based on the theoretical claim that low-ability students need more teacher guidance than high-ability students. The goals of our study were to validate a new questionnaire that measures teachers' adaptivity of beliefs and to investigate whether adaptivity scores are related to students' reading skill learning gains. We assessed 25 teachers' adaptivity of constructivist and direct-transmissive beliefs and 451 students' reading fluency and reading comprehension at eight points of measurement over one school year. Psychometric assessment of the belief adaptivity questionnaire revealed that the measure is valid and reliable. Thereby, adaptivity was found in a way that teachers' constructivist beliefs were lower when referring to low-ability compared to high-ability students and that direct-transmissive beliefs were higher for low-ability compared to high-ability students. Three-level latent growth curve modeling showed that teachers' adaptivity of beliefs affected students' reading comprehension positively but did not affect reading fluency. We propose that adaptivity should be included as a facet of teaching effectiveness in the research on teacher beliefs.