Sma- and Mad-related protein 4 (SMAD4) is the central mediator of the transforming growth factor beta signaling pathway and is closely related to mammalian reproductive ability and the development of ovarian follicles. However, little is currently known about the role of SMAD4 in mammalian follicular granulosa cell (GC) apoptosis or its regulation by miRNAs. Here, we found that the porcine SMAD4 protein was expressed at high levels in GCs and oocytes from primary, preantral, and antral follicles, and only slightly expressed in theca cells; its expression level was down-regulated in apoptotic ovarian GCs, suggesting that SMAD4 may be involved in ovary development and selection. Overexpression and knockdown of SMAD4 increased the proliferation and apoptosis of cultured porcine GCs, respectively. In addition, the use of miRNA mimics and luciferase reporter assays revealed that miRNA-26b (miR-26b) functions as a proapoptotic factor in porcine follicular GCs by targeting the 3′-untranslated region of the SMAD4 gene. Overexpression of miR-26b in follicular GCs suppressed SMAD4 mRNA and protein levels, resulting in down-regulation of the antiapoptotic BCL-2 gene and the promotion of GC apoptosis. Furthermore, transforming growth factor beta 1 (TGF-beta1) down-regulates miR-26b expression in porcine GCs. Taken together, these data suggest that SMAD4 plays a critical role in porcine follicular GC apoptosis and follicular atresia and that miR-26b may have a proapoptotic role in GCs by regulating the expression of SMAD4 in the transforming growth factor beta signaling pathway.