Environmental Education (EE) is essential to be approached in early childhood education, since it contributes to the development of conscientious citizens. Ecotoxicology, an EE subject, comprises as one of its study areas the contamination of marine environments by plastic and microplastic. Initiatives that promote information and awareness on this topic are essential for the conservation of marine environments and protection from human impacts. Among the methodologies used to carry out activities in schools focusing on EE, didactic sequences and Problem Based Learning (PBL) are noteworthy. Therefore, the aim of this work is the development of a didactic sequence focusing on EE and Ecotoxicology, on the contamination of marine environments by plastic and microplastic, developed for the ninth grade of junior high school. The didactic sequence created herein comprises five activities, totalling 13 periods. The first activity is a 1 period talking circle, which aims to make students aware of the large amount of plastic used in their daily lives. The second activity, accomplished in 3 periods, is a field trip to the beach, where students must collect the plastic waste they find and write down it main characteristics in a form. As a third activity, students must produce a letter in 2 periods to a public authority, claiming ocean health. The fourth activity is a school newspaper, and is estimated to be produced in 6 periods. As a last activity, the teacher should perform another 1 period talking circle. In the future, the didactic sequence is hoped to be applied and its results disseminated. Finally, it is noteworthy that the production of studies like this is extremely important for the adoption of effective pedagogical practices by teachers, whose aim is the development of more critical citizens.