We provide first evidence regarding the direct impact of educational mismatch on firm productivity. To do so, we rely on representative linked employer-employee panel data for Belgium covering the period 1999-2006. Controlling for simultaneity issues, time-invariant unobserved workplace characteristics, cohort effects and dynamics in the adjustment process of productivity, we find that: i) a higher level of required education exerts a significantly positive influence on firm productivity, ii) additional years of over-education (both among young and older workers) are beneficial for firm productivity, and iii) additional years of under-education (among young workers) are detrimental for firm productivity.