Peritoneal carcinomatosis remains one of the most challenging forms of dissemination in gastric and colorectal cancer. It directly determines the disease prognosis and is highly resistant to treatment. The use of the existing therapeutic approaches is often limited by low benefit-risk ratio. It is necessary to develop innovative strategies aimed at overcoming the molecular mechanisms of tumor progression associated with RAS mutations, which play a crucial role in the carcinogenesis of both colorectal and gastric cancer. The study aimed to estimate safety and preliminary efficacy of the novel peptide RAS-GTPase inhibitor «Ing-Ras» when included in the treatment regimen of patients with stage III–IV gastric and colorectal cancer, including patients with peritoneal carcinomatosis. A total of 35 patients with the confirmed diagnosis of stage III–IV gastric and colorectal cancer were included in the study. The «Ing-Ras» drug was administered at a dose of 1.8 mg/kg twice with a 7-day interval using the Pressurized Intraperitoneal Aerosol Chemotherapy (PIPAC) technique. Primary efficacy was assessed by comparing the overall survival (OS) and progression-free survival (PFS) rates of the clinical trial patients with historical control groups. The OS and PFS rates for patients in the clinical trials were 92.1% and 84.22%, respectively, which significantly (p < 0.05) exceeded the values of these rates for the historical control group (50.8% and 65.25%). Thus, the use of «Ing-Ras» can significantly improve the treatment results for patients with advanced forms of colorectal and gastric cancer.