In an attempt to engage more students in Science, Technology, Engineering and Mathematics (STEM) subjects, schools are encouraged by STEM educators and professionals to introduce students to STEM through projects which integrate skills from each of the STEM disciplines. Because little is known about the learning environment of STEM classrooms, we developed and validated a Classroom Emotional Climate (CEC) questionnaire. Initially, the questionnaire was pilot tested with six focus groups of students from three schools to obtain feedback to incorporate into a revision of the CEC. Next, the modified CEC questionnaire was administered to 698 students participating in STEM activities in 57 classes in 20 schools. Exploratory factor analysis (principal component analysis) led to reduction of the CEC to 41 items in seven dimensions: Consolidation, Collaboration, Control, Motivation, Care, Challenge and Clarity. The structure of the CEC was then further explored using confirmatory factor analysis. Internal consistency reliability, concurrent validity (ability to differentiate between classrooms), discriminant validity (scale intercorrelations) and predictive validity (associated with student attitudes) were satisfactory. Finally, Rasch analysis of data for each dimension revealed good model fit and unidimensionality of the items describing each latent variable.