Primary malignant lymphoma of the gallbladder is extremely rare and the associated radiological findings are not well described in the literature. We report a case of malignant lymphoma in the gallbladder wall of a 78-year-old woman. Pre-operative CT and MRI showed homogeneous submucosal thickening of the gallbladder wall with a preserved mucosal surface. These unique radiological findings may be useful for diagnosing malignant lymphoma of the gallbladder.