Primary central nervous system lymphoma (PCNSL) is a rare and aggressive malignancy commonly presenting with a rapid disease course and poor prognosis. Recent clinical trials have indicated improved treatment outcomes in a highly selected patient population. However, real-world data focusing on long-term, population-based outcomes remain largely unexplored. We analyzed trends in relative survival (RS) in patients diagnosed with PCNSL in Finland from 1995 to 2018. We identified 718 PCNSL patients from the comprehensive Finnish Cancer Registry (FCR) (51% males, median age 67.8 years). For the entire cohort, 5-year overall survival (OS) and RS rates were 21% and 22%, respectively. The 2-year RS was 39% for patients younger than 75 years and 14% for older patients. A gradual increase in the 2-year RS rate was observed over successive chronological diagnostic periods. Age above 75 years at diagnosis (HR 3.65, 95% CI: 2.73-4.89) and diagnosis during a calendar period of 1995-2006 (HR 1.30, 95% CI: 1.10-1.53) were associated with a significantly increased risk of death. An increase in the number of patients diagnosed with PCNSL during the study period was confirmed, and the prognosis of patients diagnosed after the age of 75 years continues to be dismal.