Atherosclerosis is the main underlying cause of cardiovascular diseases (CVDs). Its prevention is based on the detection and treatment of traditional cardiovascular risk factors1 but often fails to identify individuals at risk for early vascular disease2. Recent research has suggested new players in the pathophysiology of atherosclerosis3, highlighting the need for alternative disease biomarkers and therapeutic targets to improve early diagnosis and therapy efficacy. Here, we identified that microbially produced imidazole propionate (ImP) is associated with the extent of atherosclerosis in mice and in two independent human cohorts. Furthermore, ImP administration to atherosclerosis-prone mice fed chow diet was sufficient to induce atherosclerosis without altering the lipid profile, and it was linked to activation of both systemic and local innate and adaptive immunity and inflammation. Specifically, we found that ImP caused atherosclerosis through Imidazoline-1 receptor (I1R) expressed in myeloid cells. Blocking this ImP/I1R axis inhibited the development of atherosclerosis induced by ImP as well as by high cholesterol diet in mice. Identification of the strong association of ImP with active atherosclerosis, along with the discovery of the contribution of the ImP/I1R axis to disease progression opens new avenues for improving the early diagnosis and personalized therapy of atherosclerosis.