Purpose <p>Prospective cohort studies analyzing the effect of plant versus animal protein intake on mortality in relatively young individuals from Mediterranean populations are limited. Moreover, a limited number of studies have performed repeated measurements of protein intake. We evaluated the relationship between plant versus animal protein intake (at baseline and at the ten-year follow-up) and mortality in the “Seguimiento Universidad de Navarra” (SUN) project.</p> Methods <p>The SUN project is a prospective, multi-purpose, dynamic cohort study of Spanish university graduates. Plant and animal protein intake were assessed using a semi-quantitative food frequency questionnaire (FFQ) previously validated in Spain. Participants were divided into quartiles based on their intake of protein. Cox regression models were used, with the first quartile serving as the reference category.</p> Results <p>A total of 17,989 subjects (10,961 women and 7028 men) were included in the analysis. During 251.363 person-years of follow-up (median follow-up time: 12 years), 460 deaths were identified. The mean age at baseline was 38 years with a standard deviation of 12 years. Participants in the highest quartile of plant protein intake had a 35% lower risk of all-cause mortality compared to those in the lowest quartile after adjusting for potential confounders [hazard ratio (HR): 0.65 (95% CI: 0.45–0.93), <i>p</i> for trend = 0.009]. No significant association was found between animal protein intake and mortality [HR: 0.94 (95% CI: 0.70–1.26); <i>p</i> for trend = 0.555] nor between plant protein intake and cardiovascular or cancer mortality.</p> Conclusion <p>Plant protein intake was inversely associated with all-cause mortality in a Mediterranean population. Animal protein intake was not associated with total mortality.</p>