Objective— Atherosclerosis is a chronic autoimmune-like disease in which lipids and fibrous elements accumulate in the arterial blood vessels. T cells are present within atherosclerotic plaques, and their activation is partially dependent on costimulatory signals, which can either provide positive or negative signals that promote T-cell activation or limit T-cell responses, respectively. T-cell immunoglobulin and mucin domain 3 (Tim-3) is a coinhibitory type 1 transmembrane protein that affects the function of several immune cells involved in atherosclerosis, such as monocytes, macrophages, effector T cells, and regulatory T cells. In the present study, we determined the role of Tim-3 in the development of atherosclerosis. Approach and Results— Western-type diet–fed low-density lipoprotein receptor–deficient (LDLr ^−/− ) mice were treated with an anti–Tim-3 antibody for 3 and 8 weeks. Anti–Tim-3 administration increased fatty streak formation with 66% and increased atherosclerotic plaque formation after 8 weeks with 35% in the aortic root and with 50% in the aortic arch. Furthermore, blockade of Tim-3 signaling increased percentages of circulating monocytes with 33% and lesional macrophages with 20%. In addition, anti–Tim-3 administration increased CD4 ⁺ T cells with 17%, enhanced their activation status, and reduced percentages of regulatory T cells with 18% and regulatory B cells with 37%. Conclusions— It is known that Tim-3 acts as a negative regulator of both innate and adaptive immune responses, and in the present study, we show that anti–Tim-3 treatment augments lesion development, accompanied by an increase in the number of monocytes/macrophages and CD4 ⁺ T cells and by decreased regulatory T cells and regulatory B cells.