The increase of seed size is of great interest in Medicago spp., to improve germination, seedling vigour and, consequently, early forage yield as well as for optimizing seeding techniques and post-seeding management. This study evaluated the effects of the ectopic expression of the AINTEGUMENTA (ANT) cDNA from Arabidopsis thaliana, under the control of the seed-specific USP promoter from Vicia faba, on seed size, germination and seedling growth in barrel medic (Medicago truncatula Gaertn.). All the transgenic T2 barrel medic lines expressing ANT produced seeds significantly larger than those of control plants. Microscopic analysis on transgenic T3 mature seeds revealed that cotyledon storage parenchyma cells were significantly larger and contained larger storage vacuoles than those of the untransformed control. Moreover, the percentage of germination was significantly higher and germination was more rapid in transgenic than in control seeds. Our results indicate that the seed-specific expression of ANT in barrel medic led to larger seeds and improved seed germination, and revealed a regulatory role for ANT in controlling seed size development.