Environmental citizenship is very important in sustainability research. The criticality of the observed environmental crisis requires capable and competent environmental citizens who can act as agents of change to achieve sustainability. This research presents the validation of the Environmental Citizenship Questionnaire (ECQ) for assessing the environmental citizenship of secondary school students. To this end, Principal Component Analysis has been performed through the use of a Confirmatory Factor Analysis. In addition, there has been a verification of sphericity and a measure of sampling adequacy using the Bartlett’s and Kaiser–Meyer–Olkin (KMO) tests, respectively. Cronbach’s Alpha, eigenvalues and percentage of variance as well as Pearson’s correlation were also estimated. Using the data of 520 students in 10th grade, the ECQ showed very good results in all measurements performed, demonstrating high internal consistency, reliability and discriminant validity. From the factor analysis were derived nine factors with 76 items in total. Cronbach’s Alpha was greater than 0.702, indicating high reliability in all factors. The possible contribution of the ECQ in different contexts and educational frames and in sustainability education is discussed.