AGAMOUS (AG) MADS-box transcription factors have been shown to play crucial roles in floral organ and fruit development in angiosperms. Here, we isolated a tomato (Solanum lycopersicum) AG MADS-box gene SlMBP3 and found that it is preferentially expressed in flowers and during early fruit developmental stages in the wild-type (WT), and in the Nr (never ripe) and rin (ripening inhibitor) mutants. Its transcripts are notably accumulated in the pistils; transcripts abundance decrease during seed and placental development, increasing again during flower development. SlMBP3-RNAi tomato plants displayed fleshy placenta without locular gel and extremely malformed seeds with no seed coat, while SlMBP3-overexpressing plants exhibited advanced liquefaction of the placenta and larger seeds. Enzymatic activities related to cell wall modification, and the contents of cell wall components and pigments were dramatically altered in the placentas of SlMBP3-RNAi compared with the WT. Alterations in these physiological features were also observed in the placentas of SlMBP3-overexpressing plants. The lignin content of mature seeds in SlMBP3-RNAi lines was markedly lower than that in the WT. RNA-seq and qRT-PCR analyses revealed that genes involved in seed development and the biosynthesis of enzymes related to cell wall modification, namely gibberellin, indole-3-acetic acid, and abscisic acid were down-regulated in the SlMBP3-RNAi lines. Taking together, our results demonstrate that SlMBP3 is involved in the regulation of placenta and seed development in tomato.