BACKGROUND: Growing concern surrounds the risk of neurodegenerative diseases in high-level collision sports, but research on Rugby Union's connection to these diseases is limited. OBJECTIVE: This study sought to examine the long-term neurodegenerative disease risk associated with participation in high-level Rugby Union ('rugby'), utilising whole-population administrative records. METHODS: This retrospective cohort study in New Zealand compared males born between 1920 and 1984 who were active in high-level (provincial or higher) rugby between 1950 and 2000 (n = 12,861) with males from the general population (n = 2,394,300), matched by age, ethnicity, and birthplace. We used Cox proportional hazards models to assess risks of Alzheimer's disease, Parkinson's disease, motor neuron disease, and other dementias, ascertained using mortality and hospitalisation records from January 1988 to June 2023. RESULTS: A higher percentage of rugby players (6.5%) than males in the general population (5.2%) developed neurodegenerative diseases, with hazard ratios indicating players showed increased risks for any neurodegenerative disease (1.22; 95% confidence interval [CI] 1.14-1.30), Alzheimer's disease (1.61; 95% CI 1.42-1.83), and other dementias (1.23; 95% CI 1.14-1.33). Significant differences were not observed for Parkinson's disease (1.05; 95% CI 0.89-1.22) and motor neuron disease (1.16; 95% CI 0.83-1.63). In general, this increased risk among players compared to the general population began around the ages of 70-79 years. Compared to the general population, small to moderate increased risks of any neurodegenerative disease were observed for a backline playing position, provincial and/or amateur players, international and/or professional players, participation in ≥ 2 years of play, and participation in five or more matches. CONCLUSIONS: High-level rugby participation amongst males in New Zealand is associated with a small to moderate increase in neurodegenerative disease rates compared to the general population.