Abstract: Detecting circulating tumor DNA (ctDNA) in skin interstitial fluid (ISF) offers a non-invasive cancer diagnostic route, though clinical translation is often hindered by low extraction efficiency and sensitivity. This study presents an integrated microneedle-electrode platform for in situ sampling and monitoring of ctDNA. Utilizing reverse iontophoresis, ctDNA is extracted from the dermal ISF and subsequently detected via a signal-amplified rGO/PPy/Au NPs electrochemical sensor. The platform facilitates quantitative detection with a broad linear range (10 fM to 1 nM) and a low detection limit of 1.92 fM. It demonstrates high specificity against base mismatches and biological interferents, alongside strong reproducibility. In vivo testing on a murine tumor model showed results significantly correlated with qPCR and ddPCR, highlighting its potential for point-of-care cancer diagnostics and disease monitoring.