High water content hydrogels are needed for various biomedical applications, including soft tissue engineering, drug delivery, and soft contact lenses, due to their prominent mass transfer efficiency, biocompatibility, and nontoxicity. However, their weak mechanical properties due to excess water content often limit their broad use in biomedical applications. Inspired by the coiled fishing tentacles of Physalia physalis, we present a high water content hydrogel fiber with stretchability that is increased by a self-coiled structure. Under aqueous conditions, the electrospun hydrophilic polyurethane yarn was found to coil by itself based on changes in its mechanical properties (Young’s modulus and shear modulus) during hydrophilic polyurethane swelling. The yarn was rapidly saturated within 1 s, finally resulting in 96.7% water content. In addition, the number of coils and spring index increased up to 800 and 1000 turns/m, respectively, as the twist density of the polyurethane yarn increased. Moreover, the polyurethane fiber showed the highest spring index (1.2) at an applied stress of 0.5 kPa. Remarkably, the coiled hydrogel fiber was stretched to 600% by the uncoiling process and further extended to 930% due to the elastic property of polyurethane. Taken together, these results show that hydrophilic polyurethane yarn fiber undergoes self-coiling by swelling under aqueous conditions, which enables it to realize high water content due to its hydrophilicity and high stretchability due to its elastic property and coiled structure. Therefore, we suggest the use of this high water content and stretchable hydrogel fiber in various medical applications, such as surgical sutures, drug delivery systems, and tissue regeneration.