Nationalism and patriotism are two interrelated, but separate, forms of national attachment that fulfill diverse psychological needs and are associated with distinct political and societal outcomes, particularly amid increasing immigration and diversity. However, little is known about how these two forms of national attachment change over time and/or throughout adulthood. The present study addresses this oversight by using cohort-sequential latent growth models across 13 annual waves of longitudinal data from a national sample of adults born in New Zealand (N = 50,305–50,408) to investigate changes in nationalism and patriotism across different birth cohorts. Results reveal that nationalism remained relatively unchanged until middle adulthood, after which it slightly increased. Conversely, patriotism appeared to increase weakly, albeit gradually, across the lifespan. Some evidence of cohort and period effects also emerged, pointing to subtle increases in patriotism and decreases in nationalism among the New Zealand population. These results are the first to demonstrate changes in nationalism and patriotism over an extended timeframe and across cohorts in a national sample.