Endovascular aneurysm repair, a treatment for abdominal aortic aneurysms, carries the risk of recurrence due to endoleaks following stent graft implantation. Since these leaks do not present with specific symptoms, regular follow-up is necessary; however, now, only imaging-based monitoring is available. This study proposes an ultrathin flexible sensor inserted endovascularly with a stent to detect type I endoleaks, which pose the highest rupture risk. A coplanar capacitive sensor was attached to the stent graft using a specially developed flexible thermal adhesive to monitor blood leakage, without compromising the stent graft's function. It seamlessly folds and unfolds with the stent, allowing implantation through catheter-based procedures. Experimental validation confirmed that the sensor does not induce blood leakage and demonstrated long-term stability and functionality under vascular and dynamic conditions. This technology enables wireless monitoring of endoleak status, facilitates timely intervention, and improves postoperative outcomes by reducing the risk of recurrent aneurysms.