Background <p>MicroRNAs that are abnormally expressed in coronary atherosclerosis (CAS) have gradually been discovered due to their unique advantages.</p> Objective <p>Assessing the value of miR-200b-3p in CAS.</p> Methods <p>The level of miR-200b-3p and RAP1B in CAS (<i>n</i> = 103) and healthy (<i>n</i> = 77) subjects was analyzed by RT-qPCR. The role of miR-200b-3p in CAS was determined through the ROC curve, correlation, and multivariate logistic regression analysis. Downstream targets of miR-200b-3p were screened by online databases. A dual-luciferase reporter assay evaluated the regulatory relationship between miR-200b-3p and RAP1B. An ox-LDL-induced human vascular smooth muscle cell (HVSMC) model was used to explore the potential mechanism of the miR-200b-3p/RAP1B axis in CAS progression.</p> Results <p>The expression of miR-200b-3p was upregulated in the CAS group compared with the HC group and showed a diagnostic value in distinguishing CAS from healthy participants. The miR-200b-3p expression was correlated with TG, TC, LDL-C, HDL-C, coronary artery stenosis and calcification, and predicted the risk of CAS development. RAP1B was a target of miR-200b-3p and negatively correlated with the level of miR-200b-3p. In ox-LDL-induced HVSMCs, upregulated miR-200b-3p expression and downregulated RAP1B expression were observed. The silence of miR-200b-3p mitigated the ox-LDL-treated HVSMC injury through suppressing cell proliferation, migration, inflammation (IL-6, TNF-α), and calcification (Ca²⁺, ALP activity) in HVSMCs by targeting RAP1B.</p> Conclusions <p>Upregulation of miR-200b-3p served as a biomarker in diagnosing CAS. Upregulated miR-200b-3p expression might be involved in CAS progression through promoting the ox-LDL-stimulated vascular smooth muscle cell (VSMC) injury via regulating RAP1B.</p>