Abstract In this paper, we focus on named entity boundary detection, which aims to detect the start and end boundaries of an entity mention in text, without predicting its type. A more accurate and robust detection approach is desired to alleviate error propagation in downstream applications, such as entity linking and fine-grained typing systems. Here, we first develop a novel entity boundary labeling approach with pointer networks, where the output dictionary size depends on the input, which is variable. Furthermore, we propose AT-BDRY, which incorporates adversarial transfer learning into an end-toend sequence labeling model to encourage domaininvariant representations. More importantly, ATBDRY can reduce domain difference in data distributions between the source and target domains, via an unsupervised transfer learning approach (i.e., no annotated target-domain data is necessary). We conduct Formal Text ? Formal Text, Formal Text ? Informal Text and ablation evaluations on five benchmark datasets. Experimental results show that AT-BDRY achieves state-of-the-art transferring performance against recent baselines