Clinically, postoperative adhesions are common and serious complications, which almost always happen after abdominal or pelvic surgery. The adhesion development process is accompanied by increased inflammatory cell infiltration and oxygen-free radical production. In this study, the naturally occurring antioxidative and anti-inflammatory compounds extracted from Turkish galls by ethyl acetate (GEA) were encapsulated into an injectable and biodegradable thermosensitive hydrogel. Antiadhesion efficacy of the barrier system (GEA-NP/H) was tested on a rat peritoneum injury-cecum abrasion model. Upon injection, the mildly viscous liquid formed a potent physical barrier over the injured cecum and peritoneum without any additional cross-linkers or light sources. Once formed, GEA-NP/H acted as a durable wound dressing for more than 5 days, as well as a sustained drug depot of GEA. The polymer hydrogel can be degraded and absorbed gradually. After 14 days, severe adhesion occurred among rats treated with normal saline and GEA-loaded nanoparticles (GEA-NP). Whereas, frequency of score 1 adhesion among the blank hydrogel group is 30%, and 90% of the rats from GEA-NP/H group exhibited no adhesion. In addition, pathological sections and scanning electron microscopy assay demonstrated that operative defects treated with GEA-NP/H suffered from mild oxidative stress and inflammatory damages at early days after injury, as well as accelerated wound healing and more mature mesothelial cell deposition at the 14th day in contrast to the blank hydrogel treatment. Therefore, the study provided an available biodegradable hydrogel barrier to effectively prevent postsurgical adhesion.