The knowledge and control of invasive species are essential in the management of marine ecosystems. The Mediterranean Sea is one of the marine areas with the highest number of invasive species detected, most of them invertebrates. On a smaller scale, colonization by non-native species in coastal lagoons depends directly on their connectivity with the adjacent sea and on the changes in their environmental conditions induced mainly by human interventions. In this study, the cirratulid species Timarete caribous Grube, 1859, native from the Western and Central Atlantic Ocean, was detected for the first time in a Western Mediterranean coastal lagoon, the Mar Menor. It has been found in dense aggregates on both artificial and natural rocky substrates, mainly in the most important communication channel with the Mediterranean Sea. Following the evolution of this NIS will be important for understanding the dynamic and spread of the species and its possible effects on the indigenous biological communities.