Purpose: While early treatment of posterior uveal melanoma can save the eye, the effect of early treatment on survival remains unknown. Therefore, we aimed to determine whether the tumour size at diagnosis has changed over time, and if this has affected survival rates of patients with posterior uveal melanoma in Denmark. Methods: Nationwide retrospective cohort study linking data from registry-based resources to data from clinical charts and pathology records. Including all Danish patients diagnosed with posterior uveal melanoma from 1943 to 2017. Incidence rates were estimated as annual percentage change (APC) overall and by American Joint Committee on Cancer (AJCC) tumour sizes. The age-period-cohort model was applied to estimate the relative risk of calendar period. The cox proportional hazards model, relative survival Kaplan–Meier curves and cumulative incidence curves were applied to estimate the effect of calendar period on survival. Results: An overall increase in incidence rate of uveal melanoma was found (APC = 0.25%, 0.08–0.42; 95% CI). This was due to increasing incidence rate of AJCC T1 + T2 tumours (APC = 0.97%, 0.57–1.37; 95% CI), whereas no increase in incidence rates of AJCC T3 + T4 tumours was found (APC = −0.01%, −0.26 to 0.25; 95% CI). The disease-specific survival improved with calendar period for all tumour sizes (HR = 0.988; 0.984–0.993; 95% CI). Conclusion: Increasing incidence rate and improved survival rate for uveal melanoma was found concordantly with a decrease in tumour size during a 70-year period.