This chapter focuses on the relationship between borders and belonging in marine worlds, paying special attention to debates concerning marine invasive species. Within a number of natural and social science disciplines, studies of invasive species are both long-established and continuing to grow and change. Major research into marine ‘invasions’ has come later than terrestrial studies, so critical social scholarship in invasive species (or bioinvasion) remain largely (though not exclusively) landlocked. Where studies are watery, they are of fresh, rather than salt water and ‘interior’ rather than focusing on ‘exterior’ liquid zones. This chapter has two aims. It firstly identifies invasive species as a topical issue that is marginal in critical discussions of ocean bordering/belonging. It secondly puts forwards three ways in which bioinvasion, bordering and belonging can be better understood, through (1) questions of definition and meaning; (2) an attention to the mobilities and materialities enrolled within processes of bioinvasion—the movement of marine species in global circuits; and (3) how borders are enacted in view of invasive species via biosecurity measures. The chapter aims to disrupt conventional narratives of marine belonging, showing a biological world that is ‘stitched’ together.