Although educational assessment is one of the main responsibilities of the teachers, there has been little research designed to examine ethical principles in the daily classroom assessment practices of the teachers. Using a descriptive survey research design, the purposes of the current study were to develop a measure of the teachers’ adherence to the ethical principles in educational assessment and identify teachers’ characteristics associated with it. Participants were 3557 teachers teaching grades 5 to 12 in public schools across all educational governorates in Oman. Principal components analysis of the teachers’ responses to an 11-items survey revealed three dimensions of the ethical principles in educational assessment: confidentiality, test integrity, and transparency. Internal consistency reliability ranged from .64 to .78. The correlations among the dimensions ranged from .19 to .32. Construct validity was evidenced by the statistically significant positive low correlation of .10 between the dimensions and knowledge of educational assessment ethics. Multivariate analyses of variance revealed statistically significant differences in levels of adherence to the ethical principles in educational assessment among teachers with respect to gender, educational qualification, teaching subject, teaching experience, and training in educational assessment. It was concluded that the measure developed in this study has the potential to provide educators and researchers with valuable information to understand teachers’ adherence to the ethical principles in educational assessment.