Increased expression of bone morphogenetic proteins (BMPs) in several tissues is associated with inflammation and type 2 diabetes mellitus. BMP2 and BMP4 mRNA expression is increased in pancreatic islets from db/db mice and β-cell proliferation and function are inhibited by BMP4. The effect of BMPs on α-cells is currently unknown. Here, we investigate the effects of BMP4 on mouse and human α-cells in vitro. The effects of BMP4 on α-cell proliferation and function were investigated in islets isolated from male mice and from human donors, and in α-TC1–6 cells. The effects of BMP4 on α-cell function were assessed by determination of glucagon secretion and gene expression. Treatment with BMP4 for 24–96 hours inhibited glucagon secretion in a time-dependent manner in mouse and human islets. Glucagon content, preproglucagon and aristaless related homeobox mRNA expression were reduced after incubation with BMP4 in mouse islets, but not in human islets. The percentage of proliferating α-cells was reduced from 7.3 % to 0.2 % in mouse islets incubated with BMP4. α-cell proliferation in human islets ranged from 0 to 11.8 %, and BMP4 was found to inhibit proliferation of α-cells from all donors when proliferation was present. In agreement with the observations in primary islets, BMP4 decreased glucagon content, preproglucagon, and aristaless related homeobox mRNA expression in α-TC1–6 cells. Our findings suggest that BMP4 has an inhibitory role on glucagon secretion, α-cell growth, and expression of genes maintaining α-cell identity.