This study aimed to apply finite element analysis (FEA) to simulate the oral environment of a patient with an implant-supported overdenture prosthesis. A 3D mandibular model was created for a 45-year-old female patient from CT images, with mucosal thickness measured at 3 mm on average using ultrasonography. The overdenture prosthesis was scanned with an intraoral scanner and placed onto the 3D mandibular model. Displacement of the prosthesis and von Mises stress values of the implant, surrounding bone tissue, implant-prosthesis components, and mucosa were calculated under various masticatory loads. The study found that anterior mastication generated higher stress compared to unilateral and bilateral mastication. The addition of a third implant significantly reduced stress and improved prosthesis stability, particularly during bilateral mastication.