Damselfishes (Pomacentridae) are one of the most behaviourally diverse, colourful and species rich reef fish families. One remarkable characteristic of damselfishes is that they communicate in ultraviolet (UV) light. Not only are they sensitive to UV they are also prone to have UV-reflective colours and patterns enabling social signalling. Using more than 50 species, we aimed to uncover the evolutionary history of UV colour and UV vision in damselfishes. We find that all damselfishes had UV-transmitting lenses, expressed the UV-sensitive SWS1 opsin gene, and most displayed UV-reflective patterns and colours. We find evidence for several tuning events across the radiation, and while SWS1 gene duplications are generally very rare amongst teleosts, our phylogenetic reconstructions uncovered two independent duplication events, one close to the base of the most species-rich clade in the subfamily Pomacentrinae, and one in an individual Chromis species. Using amino acid comparisons, we found that known spectral tuning sites were altered several times in parallel across the damselfish radiation (through sequence change and duplication followed by sequence change), causing repeated shifts in peak spectral absorbance of around 10 nm. Pomacentrine damselfishes expressed either one or both copies of SWS1, likely to further finetune UV-signal detection and differentiation. This highly advanced and modified UV vision among damselfishes, in particular the duplication of SWS1 among Pomacentrinae, might be seen as a key evolutionary innovation that facilitated the evolution of the exuberant variety of UV reflectance traits and possibly the diversification of this coral reef fish lineage.