National attention on whole-of-school approaches to decrease children’s sedentary behavior and increase physical activity includes movement integration (MI) in classrooms. The purpose of this study was to describe instrument development, reliability, and validity of the System for Observing Student Movement in Academic Routines and Transitions (SOSMART), designed to assess MI in elementary classrooms. An a priori conceptual framework was developed based on existing literature. The framework was expanded/refined using videos from elementary classrooms and a Delphi survey. The survey, sent to 85 experts, yielded a 38% response rate. The final system includes 11 MI variables (three categories of teacher variables, two categories of student variables) and uses a 20-second continuous interval recording format. Reliability and validity data were collected in 12 classrooms across four elementary schools. Instrument reliability was tested using interval-by-interval percentage agreement for each category. Construct validity was tested by estimating multilevel random effects logistic regression models comparing student accelerometer derived activity with the presence/absence of each MI variable. Intraobserver reliability resulted in 97.5% agreement and exceeded 80% on all variables. Construct validity was supported for 8 out of 11 MI variables. SOSMART can provide valid, reliable, and objective data about MI in elementary schools.