Objective To analyze the clinical and imaging characteristics of diffuse-type giant cell tumor of tendon sheath (D-GCTTS). Methods Clinical and imaging data from 18 patients with pathologically confirmed D-GCTTS were retrospectively analyzed. X-ray and MRI examination were performed on all cases. The age of the patients ranged from 19 to 52 years with an average age of 29 years. Results Six tumors occurred in knee, six in ankle, three in foot and three in hand. On X-ray films, all cases showed local soft tissue mass. There was no visible calcification in the soft tissue. Invasion of adjacent bone was identified in 15 of them. On MRI, all lesions appeared as lobulated soft tissue masses with ill-defined margin. All lesions displayed isointense and hypointense signal on T1WI and heterogeneous appearance on T2WI, mainly isointense and hypointense signal mixed with hyperintense signal. Characteristic low signal intensity areas where hemosiderin was deposited in the different sequences were identified in all cases. After the injection of GdDTPA, all tumors showed heterogeneous and strong enhancement. Twelve cases showed medullary invasion of the adjacent bone, three cases showed pressure erosion of the cortex, and three showed normal of the adjacent bone. Conclusion Characteristic imaging and clinical features of D-GCTTS is very valuable for the diagnosis and treatment of this disease.