Improvements in ingestible electronics with the capacity to sense physiologic and pathophysiologic states have transformed the standard of care for patients. Yet despite advances in device development, significant risks associated with solid, non-flexible gastrointestinal transiting systems remain. Here, we disclose an ingestible, flexible piezoelectric device that senses mechanical deformation within the gastric cavity. We demonstrate the capabilities of the sensor in both in vitro and ex vivo simulated gastric models, quantified its key behaviors in the GI tract by using computational modeling, and validated its functionality in awake and ambulating swine. Our piezoelectric devices can safely sense mechanical variations and harvest mechanical energy inside the gastrointestinal tract for diagnosing and treating motility disorders and for monitoring ingestion in bariatric applications.