Molecular electronics originally proposed that small molecules sandwiched between electrodes would accomplish electronic functions and enable to reach ultimate scaling. However, so far, functional molecular devices have been only demonstrated at low frequency. Here, we demonstrate molecular diodes operating up to 17.8 GHz. DC (direct current) and RF (radio frequency) properties were simultaneously measured on a large array of molecular junctions composed of gold nanocrystal electrodes, ferrocenyl undecanethiol molecules, and the tip of an interferometric scanning microwave microscope. The present nanometer-scale molecular diodes offer a current density increase by several orders of magnitude compared to that of micrometer-scale molecular diodes, allowing RF operation. The measured S11 parameters show a diode rectification ratio of 12 dB which is linked to the rectification behavior of the DC conductance. From the RF measurements, we extrapolate a cut-off frequency of 520 GHz. A comparison with the silicon RFSchottky diodes architecture suggests that the RF-molecular diodes are extremely attractive for scaling and high-frequency operation.