State-of-the-art microfluidic devices for ultra-fast, electronic, and label-free sensing of single particles struggle to detect nanoscale objects due to limited sensitivity at high measurement bandwidths. To increase the sensitivity, we designed a novel coupling-based sensor operating at 20 GHz. The novelty consists of a coplanar waveguide (CPW) resonator capacitively coupled via a fluidic channel to the feedline. We compare the coupling theory with finite element method (FEM) simulations, which allows us to fine-tune the sensor’s geometry for an optimal coupling coefficient of 0.33. Furthermore, we simulated the sensor’s response to particles in the sensing volume and compared its performance with that of conventional tank circuit (TC) sensors. We successfully validated the coupling concept by comparing a manufactured sensor with simulations and finally we demonstrated the sensor’s exceptional sensitivity through the in-flow detection of 200 nm-sized polystyrene beads, achieving a signal-to-noise (S/N) ratio of up to 417. To our knowledge, these are, to date, the smallest detected individual particle at radio frequencies with the highest S/N ratio. Therefore, our study is a substantial step toward detecting sub-100 nm particles, enabling the high-throughput classification of virions, proteins, or DNA based on their dielectric properties.