Malignant ascites indicate the presence of malignant cells in the peritoneal cavity that lower patient survival and reduce quality of life. Current chemotherapy regimens suffer from the dilution of ascites and rapid metabolism limiting their therapeutic efficacy. The storage and sustained release of drugs at the tumor site represents a promising strategy to improve drug efficacy. The aim of this study was to develop injectable hyaluronic acid hydrogel containing polymeric gemcitabine nanoparticles and cisplatin for the local treatment of malignant ascites through a dual sustained drug release pattern. Cell uptake assays showed that the drug-loaded nanoparticles readily entered tumor cells. Apoptosis and cell cycle analysis showed that the hydrogel system could enhance tumor cell apoptosis and arrest more cells in the G1 phase. In vivo experiments indicated that mice treated with the drug-loaded hydrogels manifested the most significant efficacy in ascites volume, tumor nodules, body weight, abdominal circumference, and survival. The expression of Ki-67 and CD31 also significantly decreased compared with other groups, indicative of anti-tumor activity. In addition, intraperitoneal administration of the hydrogel system led to no significant damage to vital organs. These findings confirm the clinical potential of the drug-loaded hydrogel system for the treatment of malignant ascites.