Environmental security has first been mentioned in a UN publication in 1987. Since then, the UN – its agencies but also as a forum for Member States – has been increasingly involved in the field of environment and security. However, very few studies try to understand the activities of the organization on that matter; this paper intends to analyze them using the securitization theories. From these theories' perspective, environmental security primarily constitutes the example of a semantic evolution in International Relations: from a scientific issue, it progressively goes to a political and a security issue. This paper intends to explore the social construction of the environment as a legitimate and relevant issue on the UN security agenda. Based on a specific empirical case study – participant observation and semi-structured interviews –, it also aims to complete securitization theories that have been criticized and revised since the first studies by the Copenhagen School. This example will especially seek to rethink securitization and politicization, showing that UN activities could reveal an attempt of a securitization without politicization framing the environment as an apolitical issue that requires a non-politicized scientific approach.