Malignant ascites affects approximately 10% of patients with gastric cancer (GC), and poses significant difficulties for both patients and clinicians. In addition to the dismal general condition of affected patients and the diversity of associated complications such as jaundice and ileus, problems in assessing scattered tumors have hampered the expansion of clinical trials for this condition. However, the accumulation of reported studies is starting to indicate that the weak response to treatment in GC patients with malignant ascites is more relevant to their poor prognosis rather than to the ascites volume at diagnosis. Therefore, precise assessment of initial state of ascites, repetitive evaluation of treatment efficacy, selection of suitable treatment, and swift transition to other treatment options as needed are paramount to maximizing patient benefit. Accurately determining ascites volume is the crucial first step in clinically treating a patient with malignant ascites. Ultrasonography is commonly used to identify the existence of ascites, and several methods have been proposed to estimate ascites volume. Reportedly, the sum of the depth of ascites at five points (named "five-point method") on three panels of computed tomography images is well correlated to the actual ascites volume and/or abdominal girth. This method is already suited to repetitive assessment due to its convenience compared to the conventional volume rendering method. Meanwhile, a new concept, "Clinical Benefit Response in GC (CBR-GC)", was recently introduced to measure the efficacy of chemotherapy for malignant ascites of GC. CBR-GC is a simple and reliable patient-oriented evaluation system based on changes in performance status and ascites, and is expected to become an important clinical endpoint in future clinical trials. The principal of treatment for GC patients with ascites is palliation and prevention of ascites-related symptoms. The treatment options are various, including a standard treatment based on the available guidelines, cytoreductive surgery with hyperthermic intraperitoneal chemotherapy (HIPEC), laparoscopic HIPEC alone, intravenous chemotherapy, intraperitoneal chemotherapy, and molecular targeting therapy. Although each treatment option is valid, further research is imperative to establish the optimal choice for each patient.