OBJECTIVE: To evaluate the effectiveness of preventing metastasis for each major treatment modality for iris melanoma. DESIGN: Retrospective case series. PARTICIPANTS: Three hundred consecutive eyes with iris melanoma at a single tertiary referral centre for ocular oncology. METHODS: Retrospective analysis of eyes with iris melanoma, both with (n = 69 eyes) and without (n = 231 eyes) ciliary body extension, was undertaken for metastasis-free survival at 5, 10, and 20 years based on type of treatment, including globe-sparing surgical resection (n = 169 eyes), plaque radiotherapy (n = 74 eyes), or enucleation (n = 57 eyes). RESULTS: For the total population, 5-, 10-, and 20-year metastasis-free survival rates were 95%, 93%, and 87%, respectively, and there was no difference in metastatic rates for tumours with versus without ciliary body extension (p = 0.95). Noninferiority was demonstrated for surgical resection and plaque radiotherapy, with metastasis-free survival rates of 98%, 97%, and 94% for surgical resection and 94%, 94%, and 89% for plaque radiotherapy (p = 0.002). The rates for globe salvage were 94%, 92%, and 90% for surgical resection and 94%, 86%, and 86% for plaque radiotherapy (p = 0.003). However, metastasis-free survival was worse in patients who underwent enucleation (86%, 67%, and NA; p < 0.001). CONCLUSIONS: Metastasis-free survival and globe salvage following plaque radiotherapy and surgical resection are not inferior to either, but eyes undergoing enucleation demonstrated a lower metastasis-free survival, likely because enucleation is performed for larger, more extensive melanomas, often with secondary glaucoma. In this analysis, iris melanoma with ciliary body involvement did not increase the risk of metastasis.