Title: A simple Training-Free Method for Rejection Option. Abstract: We present a simple yet effective method to implement the rejection option for a pre-trained classifier. Our method is based on a sound mathematical framework, enjoys good properties, and is hyperparameter free. It is lightweight, since it does not require any re-training of the network, and it is flexible, since it can be used with any model that outputs soft-probabilities. We compare our solution to state-of-the-art methods considering popular benchmarks (Cifar-10, Cifar-100, SVHN), and various models (VGG-16, DenseNet-121, ResNet-34). At evaluation time, our method, which is applied post-training to any classification model, achieves similar or better results with respect to its competitors that usually require further training and/or tuning of the models.