We report on an extremely rare case of a giant solitary fibrous tumor (SFT) of the mesentery in a 65-year-old male who was admitted to our hospital because of lower abdominal pain and abdominal fullness. Computed tomography demonstrated a well-defined solid mass of 25 × 11 cm located in the lower abdomen, which was completely resected during surgery. Histopathologically, this lesion had a heterogeneous cell population, mainly comprising spindle cells with fibrous collagen proliferation, and various other cell populations exhibiting patternless growth. Immunohistochemically, the tumor revealed strong and diffuse staining for CD34, bcl-2, and vimentin, and a high mitotic index (seven mitoses per 10 high-power fields). We diagnosed this case as an SFT of the mesentery, which is unusual according to a PubMed search that reported only nine such cases. Our case may be the largest tumor reported to date, and only one retrieved case reported recurrence, although the lesion was exceptionally large with deep invasion. Nonetheless, the lesion in our case was larger than that in the reported case of recurrence and invasive to the ileum. Since surgery, there has been no evidence of recurrence. Hence, we propose that a large SFT and high mitotic index may present risk factors for recurrence. Therefore, long-term careful follow-up is necessary in such cases, although our case exhibited few risk factors for recurrence. A follow-up at 12 months after surgery found no indications of recurrence.