Background: Cardiovascular diseases (CVDs) are the leading global cause of mortality, with vascular calcification (VC) as a major predictor of adverse outcomes. Although vascular smooth muscle cells (VSMCs) are established contributors, the role of endothelial cells (ECs), particularly via the endothelial–mesenchymal transition (EndMT) and exosome signaling, remains less defined. Objective: This study investigated whether the gut microbiota-derived metabolite Trimethylamine-N-oxide (TMAO) induces EndMT in ECs and whether exosomes from TMAO-treated ECs regulate the VSMC phenotype and calcification. Methods: Human umbilical vein endothelial cells (HUVECs) were exposed to TMAO at physiological and pathological levels (10–50 µM). EndMT markers were analyzed by Western blotting and qPCR. Exosomes were isolated, characterized, and applied to HAVSMCs in graded doses. Osteogenic and contractile markers, β-catenin signaling, and calcification were quantified. Exosomal miR-30 and miR-222 were studied. Results: TMAO triggered dose-dependent EndMT, decreasing CD31/VE-cadherin and increasing α-SMA, N-cadherin, and vimentin. Exosomes from TMAO-treated ECs reprogrammed VSMCs, downregulating contractile proteins and upregulating RUNX2, OPN, TNAP, and β-catenin, causing calcium accumulation. These exosomes displayed elevated miR-222 and reduced miR-30, changes that activated β-catenin signaling and promoted the osteogenic reprogramming of VSMCs. Conclusions: Pathophysiological TMAO levels induce EndMT and mediate the formation of exosomes, which drive the osteogenic reprogramming and calcification of VSMCs.