Seed yield of alfalfa (Medicago sativa L.) is important in determining the effective distribution of new cultivars to farmers. Many genetic and environmental factors affect seed yield. This study was conducted to explain seed yield variation induced by either environmental conditions or cultivars. We analyzed seed yield, aboveground phytomass, harvest index, and seed yield components for a set of 12 cultivars at four locations across France in each of three years. Each location × year combination was considered an environment. Seed weight, number of pods per inflorescence, number of seeds per pod, and mean seed weight were measured. Mean seed yield was 801 kg ha−1 Large variation in seed yield was found among cultivars and environments. The cultivar × environment interaction was significant. Among environments, seed yield was highly correlated with aboveground phytomass at harvest (r = 0.94) as the lowest seed yields were obtained in the seeding year. The cultivars most adapted to grazing showed the lowest seed yields. Seed yield was genetically correlated with lodging resistance (r = −0.89) and harvest index (r = 0.99). The mean harvest index was 12.7%. The seed weight per inflorescence showed a high broad‐sense heritability (0.58) and a high genetic correlation with seed yield (r = 0.91) and with harvest index (r = 0.96). Variation in seed weight per inflorescence was associated with variation in the number of seeds per pod and number of pods per inflorescence. Seed weight per inflorescence appears to have a strong genetic association with seed yield in alfalfa. Environments with high aboveground phytomass potential also have high seed yield potential.