Background: Clinical trials in patients with advanced cancer rely on a good selection of candidates based on their functional status. The Eastern Cooperative Oncology Group (ECOG) Performance Status is the most frequently used in adult patients and is based on the subjective impression of the clinician during the interview with the patient. The current use of wearable devices will allow clinicians to assess the functional status of candidates for a clinical trial in a more objective way. We aim to define an “Objective Performance Status” (OPS) by measuring the daily physical activity of patients with metastatic cancer and correlating it with the classical prognostic variables. Methods: We prospectively measured daily physical activity with a wearable device in 111 patients with metastatic cancer during the screening period for a phase 1 clinical trial to avoid treatment interference with the functional status. Daily steps/distance and sleeping hours were measured and correlated with overall survival (OS) to define the OPS. Results: Full data was available for 70 patients. Median survival was gradually shorter as physical activity decreases (HR 5.76 (95% CI 2.98-11.1) P < 0001). The cut off selected to define the OPS was determined by univariate survival analysis and showed that patients who walk less than 1200 meters per day have 5 times the risk of mortality (poor OPS). Median OS was 3.2 months (95% CI 2.5-5.8) P < 0001) for patients with poor OPS and 11.2 months for patients with good OPS (95% CI 7.2-31.3 P < 0001). Conclusions: The objective assessment of performance status (OPS) using a wearable device to monitor the physical activity in patients with cancer is an independent prognostic factor for survival and is an objective predictor of mortality.