Atherosclerosis is a serious cardiovascular disease, featuring inflammation, abnormal proliferation and migration of vascular smooth muscle cells (VSMCs). During atherosclerosis, inflammation may cause low pH. T-cell death-associated gene 8 (Tdag8) is a proton-sensing receptor, however, the role of Tdag8 in VSMCs remains unknown. This study aimed to investigate the potential effects of Tdag8 in VSMCs during atherosclerosis. We examined the expression of Tdag8 in an atherosclerotic model of high-fat-diet-fed ApoE-/- mice, while the role and mechanism of Tdag8 in phenotype transformation, proliferation and migration of VSMCs were investigated in a series of in vivo and in vitro experiments. We first found that Tdag8 expression at the mRNA and protein level was significantly increased in atherosclerotic ApoE-/- mice. Immunofluorescence staining showed that Tdag8 was primarily distributed in PCNA-positive VSMCs and the phenotype of VSMCs switching from contractile phenotype to synthetic phenotype. Additionally, the protein level of Tdag8 was upregulated in FBS-treated VSMCs. VSMCs proliferation and migration were inhibited by Tdag8 silencing and increased by Tdag8 overexpression. Further mechanistic studies showed that cAMP level was increased in Tdag8-overexpressing VSMCs and ApoE-/- mice. However, the PKA inhibitor H-89 reversed Tdag8-induced VSMC proliferation and migration. The results demonstrate that Tdag8 mediated phenotype transformation, proliferation and migration of VSMCs via the cAMP/PKA signaling pathway, thus partially contributing to atherosclerosis. Copyright © 2020. Published by Elsevier B.V.