There are ongoing concerns that ecological communities are undergoing rapid shifts towards new, unprecedented combinations of species, and freshwater systems are often considered to be particularly vulnerable to these changes. However, most assessments of ecological novelty focus on novel taxonomic compositions, ignoring the functional consequences of these changes. Here we apply a statistical method to detect dramatic and unprecedented changes in both the taxonomic and functional composition of riverine fish communities across the globe over the last five decades. Our results show that while taxonomic novelty was relatively common, affecting approximately 16% of the communities surveyed, taxonomic novelty was not commonly associated with functional novelty. Without downplaying the significance of the emergence of taxonomically novel communities, our results suggest that the functional composition of communities will sometimes – although not always – be conserved. Our study reinforces the need to study ecological novelty at both taxonomic and functional levels, and provides hope that dramatic changes in species composition may not always threaten the functional composition of freshwater fish communities.