A 36-year-old woman was diagnosed with anaplastic large cell lymphoma (ALCL) by excisional biopsy of a left frontal skin lesion. During the first cycle of chemotherapy (cyclophosphamide, doxorubicin, vincristine, and prednisolone), the patient complained of right ocular pain and inflammation. Cytologic examination using aqueous humor revealed atypical lymphocytes, suggesting intraocular ALCL involvement. Acute angle closure developed in the anterior chamber due to rapid progression of ALCL, causing pupillary block. Laser and surgical interventions were attempted but failed to relieve the pupillary block. Finally, radiation therapy resolved the pupillary block to restore the anterior chamber and normalize intraocular pressure. This is the first case in the English literature of ALCL involving the iris to cause acute secondary angle closure.