Title: Learning Semantic Similarities for Prototypical Classifiers. Abstract: Recent metric learning approaches parametrize semantic similarity measures through the use of an encoder trained along with a similarity model, which operates over pairs of representations. We extend such a setting and enable its use in tasks including multi-class classification in order to tackle known issues observed in standard classifiers such as their lack of robustness to out-of-distribution data. We do so by further learning a set of class prototypes, each one representing a particular class. Training is carried out so that each encoded example is pushed towards the prototype corresponding to its class, and test instances are assigned to the class corresponding to the prototype they are closest to. We thus provide empirical evidence showing the proposed setting is able to match object recognition performance of standard classifiers on common benchmarks, while presenting much improved robustness to adversarial examples and distribution shifts. We further show such a model is effective for tasks other than classification, including those requiring pairwise comparisons such as verification and retrieval. Finally, we discuss a simple scheme for few-shot learning of new classes where only the set of prototypes needs to be updated, yielding competitive performance.