Background: Positive peritoneal cytology is classified as M1 disease in gastric and pancreatic cancer. While peritoneal cytology is typically obtained by laparoscopic peritoneal lavage, this study sought to examine the feasibility and safety of performing this percutaneously, with monitored anesthesia care and in combination with other diagnostic procedures to condense and expedite the staging process. Methods: Patients with gastric or pancreatic cancer scheduled for laparoscopy with peritoneal lavage were prospectively enrolled to undergo intraoperative percutaneous peritoneal lavage prior to laparoscopic peritoneal lavage. Saline was infused through a percutaneously-inserted catheter and fluid was collected for peritoneal cytology. Three-quadrant washings collected during laparoscopy were also sent for peritoneal cytology. The primary outcome was to evaluate the sensitivity and specificity of percutaneous peritoneal lavage for detecting positive peritoneal cytology compared with the gold standard of laparoscopic peritoneal lavage, while the secondary outcome was to determine safety. Results: Percutaneous peritoneal lavage was successfully performed in 70 of 76 patients (92%). Ten of 48 gastric cancer patients (21%) and three of 22 pancreatic cancer patients (14%) had positive percutaneous and laparoscopic peritoneal cytology. Two additional gastric cancer patients had positive laparoscopic peritoneal cytology only. Sensitivity and specificity of percutaneous peritoneal lavage compared with laparoscopic peritoneal lavage were 87% and 100%, respectively. No complications occurred with percutaneous peritoneal lavage. Conclusions: Percutaneous peritoneal lavage is a safe and effective minimally invasive alternative to laparoscopic peritoneal lavage for the diagnosis of metastatic gastric and pancreatic cancer. It is possible this can be utilized in an outpatient setting, such as during endoscopy, to allow for earlier diagnosis of M1 disease and decreased time to appropriate treatment.