Adaptive radiations are typically triggered when a lineage encounters a significant range of open niche space (ecological opportunity), stemming from i) colonization of new areas, ii) extinction of competitors, or iii) key innovations. The most well-known of these is the colonization of new areas, either through geographic dispersal or the invasion of a novel ecological habitats. One aspect of ecological opportunity that has rarely been studied, however, is whether the existence of potential competitors may act to limit evolutionary diversification in newly colonized adaptive zones. Here, we show that in multiple geographically independent reinvasions of freshwaters by marine Sea Catfishes (Ariidae), rates of diversification (estimated as a function of morphological disparity and cladogenesis) have been constrained by pre-existing high diversity freshwater lineages. Only one region (Australia-New Guinea), characterized by an otherwise-depauperate freshwater fauna, has an ariid invasion gained any substantial traction. This is true at both regional and community scales, suggesting that competitive constraints may be an important factor for adaptive radiation.