The dentition of fishes can be quite striking and is often correlated with a specific diet. Combtooth blennies have long incisiform oral teeth, unlike most actinopterygians. It has been suggested that the long tooth morphology is an adaptation for detritivory, but given the diversity of diets (detritus, coral polyps, polychaetes, and pieces of other fishes), are blenny teeth indeed monomorphic? Or does tooth variation associated with diet still exist at this extreme? To explore tooth and diet diversification, we used a new phylogenetic hypothesis of Blenniidae, measured tooth shape, number, and mode of attachment, and quantified blenniid diet. The ancestral diet of blennies contained detritus and diversified into many different diets, including almost exclusively detritivory. Our results reveal a dental cline that may be constrained by tooth shape, but has not prevented diet diversification. Ancestral state reconstruction of tooth morphologies suggests that the ancestor of blennies had many unattached teeth and featured transitions to fewer attached teeth, with several transitions back to attached or unattached teeth. The dentition of blenniids is not monotypic; rather it is diverse and small changes in tooth shape are accompanied by changes in size, number, attachment, and often diet.