The fibrous cap of atherosclerotic plaques is essential for the plaque stability. Rupture of the fibrous cap leads to heart attacks and strokes, and causes tens of millions of deaths globally every year. Identifying and understanding the cellular origins and plasticity of the fibrous cap is critical to developing therapeutic strategies to stabilize the atherosclerotic plaques. Utilizing the lineage tracing mouse model, we demonstrated that fibrous cap cells arise from a predefined population of cells that expresses Notch3 at baseline. After pulse-labeling the Notch3 CreERT2 ; ROSA lsltdTomato ; Apoe -/- mice with tamoxifen before high fat diet and then feeding them with high fat diet for 16 weeks, Notch3 lineage traced cells stain positive for SMC markers (TAGLN, CNN1) and are nearly exclusively found at the fibrous cap in multiple atherosclerotic prone beds. Furthermore, Notch3-lineage traced SMCs and chondrogenic SMCs are mutually exclusive in the plaque, as demonstrated by the minimal overlap of tdTomato with chondrogenic SMC markers, including Col2a1 and Sox9. Not all fibrous caps are marked, which could reflect poor Cre efficiency or new cap cells that form later. Increase tamoxifen treatment frequencing throughout high fat diet (every 10 days) significantly improved labeling efficiency and led to fully labeled lesion cap of the whole aortic root plaque. Consistently, Notch3 + cells are committed to fibrous cap formation and excluded from the calcified portions of the lesion and acellular core. Collectively, these lineage tracing studies highlight previously unrecognized medial SMC heterogeneity in healthy vessels. Unique Notch3 + populations of SMCs in normal media are fated to form the lesion cap. Once the Notch3 program is turned on, cells are locked into a fibrous cap fate and do not give rise to osteochondrogenic SMCs. Importantly, Notch3 CreERT2 mice can be used as a cap-specific genetic manipulation tool.