Intraocular malignant mesenchymal tumors are very rare, and only a few case reports of such primary and metastatic tumors have been reported. We report a case of a malignant mesenchymoma involving the entire uveal tract. A 21-year-old woman presented with a tumor on the whole iris of the right eye, which caused intractable glaucoma. Upon enucleation of the eye, a very anaplastic tumor was found to occupy the whole uveal tract; its features were compatible with a tumor of mesenchymal origin, including rhabdomyosarcomatous and liposarcomatous characteristics. Choroidal osteoma was a coincidental finding. The histologic findings of the tumor were of two types of malignant mesenchymal tumors, and therefore the diagnosis of malignant mesenchymoma was made. This is to our knowledge the first tumor of its kind to be reported intraocularly.