This study examined Chinese and US middle-school science teachers' perceptions of autonomy support. Previous research has documented the link between teachers' perceptions of autonomy and the use of student-oriented teaching practices for US teachers. But is not clear how the perception of autonomy may differ for teachers from different cultures or more specifically how motivation factors differ across cultures. The survey measured teachers' motivation, perceptions of constraints at work, perceptions of students' motivation, and level of autonomy support for students. Exploratory factor analysis of responses for the combined teacher sample (n = 201) was carried out for each of the survey assessments. Significance testing for Chinese (n = 107) and US (n = 94) teachers revealed significant differences in teachers' motivation and perceptions of constraints at work and no significant differences for perceptions of students' motivation or their level of autonomy support for students. Chinese teachers' perceptions of constraints at work, work motivation, and perceptions of student motivation were found to significantly predict teachers' autonomy support. For the US teachers, teacher motivation was the only significant predictor of teachers' autonomy support. A sub-sample of teachers (n = 19) was interviewed and results showed that teachers in both countries reported that autonomy was important to their motivation and the quality of science instruction they provided to students. The primary constraints on teaching reported by the US teachers related to materials and laboratory space while the Chinese teachers reported constraints related to the science curriculum and standards.