An asymptomatic 40-year-old woman was found to have gallbladder wall thickening during a routine medical examination. There was no medical history except for uveitis with recent blurred vision. She was taking no medications and had no history of allergy. Her vital signs were unremarkable. On physical examination, cervical and inguinal lymphadenopathy, parotid gland enlargement, and an erythematous skin rash were detected. Soluble interleukin-2 receptor and angiotensin-converting enzyme concentrations were 2431 U/mL (normal range 121–613) and 27.6 U/L (normal range 8.3–21.4), respectively. Other laboratory data, including tumor markers such as carcinoembryonic antigen (CEA) and carbohydrate antigen 19-9 (CA19-9), were within the normal range. Abdominal ultrasonography revealed a hypoechoic mass (30-mm diameter) with no posterior echo enhancement on the abdominal side of the body of the gallbladder. The hypoechoic mass had slightly irregular borders, was elliptical in shape, and was almost uniform in consistency. In addition, another hypoechoic mass (20-mm diameter) with a clear circular border was noted in the hepatoduodenal ligament and suspected of being an enlarged lymph node. Contrast-enhanced computed tomography (CECT) revealed a mass lesion of the gallbladder wall with a contrast effect. Also, bilateral parotid gland enlargement and enlargement of multiple lymph nodes around the right supraclavicular, mediastinum, lower thoracic para-esophageal, common hepatic artery, and hepatoduodenal ligament were detectable. These lymph nodes showed uptake on positron emission tomography-computed tomography (PET-CT); however, the gallbladder lesion did not. This lesion revealed hypointensity on T1 and T2 images and no diffusion limitation on magnetic resonance imaging (MRI). Transbronchial lung biopsy (TBLB) and bronchoalveolar lavage (BAL) were performed. Finally, our patient was diagnosed as having systemic sarcoidosis by biopsy. She was prescribed prednisolone (PSL) 25 mg/day for the systemic sarcoidosis, resulting in complete resolution or reduction in size of all mass lesions except for the gallbladder lesion. Endoscopic ultrasonography (EUS) of the gallbladder lesion showed that the gallbladder lumen was intact and did not identify any mucosal lesions. It was concluded that she had a steroid-resistant lymph node in the gallbladder wall. Excision of the gallbladder lesion by laparoscopic cholecystectomy was therefore performed. The patient was discharged on postoperative day 5 with no postoperative complications. The resected specimen measured 37 × 17 mm, was yellowish-white in color, and contained an elastic hard mass under the gallbladder serosa. Histopathological examination showed that the tumor was composed of spindle cells in an irregular palisade arrangement and Verocay body formation with no evidence of malignancy. The spindle cells were positive for S-100 protein on immunostaining. In addition, non-necrotic granulation, which is consistent with sarcoidosis, was found in part of the lamina propria. Finally, the gallbladder lesion was diagnosed as a schwannoma. Five months after surgery, the PSL dosage had been gradually reduced to 5 mg/day and the patient was doing well with no evidence of recurrence.