Title: Semi-Supervised Single Domain Generalization with Label-Free Adversarial Data Augmentation. Abstract: Domain generalization (DG) has attracted increasing attention recently, as it seeks to improve the generalization ability of visual recognition models to unseen target domains. DG leverages multiple source domains for model training, while single domain generalization (SDG) further restricts such setting by exploiting only a single source domain. Nevertheless, both DG and SDG assume that the source domains are fully labeled, which might not be practical in many real world scenarios. In this paper, we present a new problem, i.e., semi-supervised single domain generalization (SS-SDG), which aims to train a model with a partially labeled single source domain to generalize to multiple unseen testing domains. We propose an effective framework to address this problem. In particular, we design a label-free adversarial data augmentation strategy to diversify the source domain, and propose a novel multi-pair FixMatch loss to generalize classifiers to unseen testing domains. Extensive experiments on OfficeHome, PACS and DomainNet20 datasets show that our method surpasses the latest SDG and semi-supervised methods. Moreover, on PACS and DomainNet20, our method approaches the fully supervised ERM upper bound within $5\%$ gap, but only uses less than $8\%$ of the labels.