The introduction of marine non-indigenous species is a global concern. Addressing the problem is made more challenging by the deficiency of data for non-indigenous estuarine and marine organisms, especially within Southeast Asia. In Singapore, three of the most important categories of marine invasion pathways are present: 1) shipping-related pathways, 2) commercial culture and fisheries, and 3) the ornamental trade. In this study, we conducted an exhaustive review to determine the status of and estuarine non-indigenous species in Singapore. A total of 17 non-indigenous species are established in estuarine and marine environments in Singapore. Commercially important non-indigenous species, which are imported for trade but are not known to be present in the environment, were also reviewed. We concluded by highlighting research directions aimed towards setting up a meaningful management framework for non-indigenous species.