Aims: Ruthenium-106 brachytherapy is an effective method for treating small to medium uveal melanomas. The purpose of this study was to examine its effectiveness and safety in the management of thick posterior uveal melanoma (apical height ≥8.0 mm) and to compare it with enucleation. Methods: 126 consecutive patients with thick uveal melanoma were included. 63 patients treated with Ruthenium-106 brachytherapy were compared with 63 patients treated with enucleation. The main outcome measures were visual acuity, eye retention, local recurrence, metastases, all-cause mortality and melanoma-related mortality. Results: Patients treated with brachytherapy were significantly younger and had significantly smaller tumours, compared with patients treated with enucleation. Tumour thickness in the brachytherapy group was 9.3 (SD 0.9) mm compared with 12.2 (1.9) mm in the enucleation group. The 5- and 10-year melanoma-related mortality was 20.5% and 46.2% for brachytherapy patients and 28.1% and 44.0% for enucleation patients (p = 0.6 and p = 0.9). When comparing 15 brachytherapy with 15 matched enucleation patients, the 5-year melanoma-related mortalities were similar, 28.6% and 33.3% respectively (p = 0.7). Complications associated with brachytherapy included tumour regrowth (n = 15), scleral melt (n = 3), neovascular glaucoma (n = 5) and vitreous haemorrhage (n = 3). In the brachytherapy group, no significant difference in survival was noted between patients who did and did not develop local recurrence (p = 0.9). Of the eyes that were initially treated with brachytherapy, 71.4% were saved from enucleation. Of these, 70.8% had a final visual acuity of 20/200 or better. Conclusions: Ruthenium-106 brachytherapy is an alternative to enucleation in some thick posterior uveal melanomas.