Robotic devices are being employed in gastrointestinal endoscopy procedures for diagnostics and surgery. However, force measurement, a very important factor for control and haptic feedback, becomes very challenging due to the strict size limitation of such devices and the flexible nature of the endoscope. This paper focuses on the modeling of the interior stomach wall for tool–tissue interactions from two perspectives: (1) If the interaction force between the robotic tool and the tissue can be measured, we utilize the force information to estimate the mechanical property of the stomach wall in real-time; given the force and position information, we would derive mathematically the only system model that can guarantee identifiability under arbitrary manipulation; (2) in the worst case scenario where force measurement is not available, we propose a viscoelastic model to restore force information solely based on position and motion information available from the robot. Ex-vivo experiments were performed on porcine stomach specimens to demonstrate the performance of the proposed models. Based on these findings, generalization and implementations of the modeling in real-time applications were discussed.