Background Tumor regression of uveal melanomas (UMs) after radiotherapy has been reported as a valuable prognostic factor for metastasis and metastatic death. But its effect on prognosis is questionable. The purpose of this study was to summarize the regression features of uveal melanoma after iodine-125 plaque brachytherapy and the relationship with prognosis. Methods Adult uveal melanoma patients who only received iodine-125 plaque brachytherapy between December 2009 and March 2018 at the Beijing Tongren Hospital, Capital Medical University were enrolled in this study. The regression rate was calculated as the percent change in tumor height, and each eye was classified for four main regression patterns: Decrease (D), Stable (S), Others (O), and Increase (I), according to the trend of height change. Statistical analysis was performed using one-way ANOVA and chi-square test, univariate and multivariate logistic regression, and Kaplan-Meier analysis. Results A total of 139 patients was included in the study. The median follow-up was 35 months. Regression patterns status was pattern D in 65 tumors (46.8%), pattern S in 50 tumors (36.0%), pattern O in 6 tumors (4.3%), and pattern I in 18 tumors (12.9%). Reductions of tumor mean height for each follow-up visit were 5.26% (3 months), 10.66% (6 months), 9.37% (12 months), and 14.68% (18 months). A comparison (D vs. S vs. O vs. I) revealed the preoperative height of pattern I was significantly lower than the pattern D, S and O (mean: 7.24 vs. 7.30 vs. 6.77 vs. 5.09 mm, respectively; P = 0.037). LBD (largest basal diameter) was strongly associated with the metastasis (P = 0.03). However, an association between the tumor regression and subsequent melanoma-related metastasis and mortality could not be confirmed (P = 0.66 and P = 0.27, respectively). The tumor regression rate increased with increasing tumor height (P = 0.04) and decreased with increasing of LBD (P = 0.01). Conclusion Our study showed a lack of association between the prognosis and the regression of uveal melanomas following I-125 plaque radiotherapy. The LBD and original height of the tumor have predictive value in tumor regression rate, and LBD was positively associated with metastasis.