A single crystal chemical vapor deposition diamond-based microdosimeter prototype featuring an array of micro-sensitive volumes (μSVs) and surrounded by a so-called guard ring (GR) electrode has been fabricated using various microfabrication techniques available at Diamond Sensors Laboratory of CEA, Saclay. The GR microdosimeter was irradiated by a raster scanning method with 2 MeV proton microbeams. The charge transport properties of the GR sensor were determined with sub-micron spatial resolution by measuring the charge collection efficiency (CCE), the μSV geometry, and the pulse-height spectra. The response of the microdosimeter showed a well-defined and homogeneously active μSV. Appropriate biasing of the μSV structures led toward a full CCE for protons with lineal energies of ∼46 keV/μm. This shows the GR microdosimeter's great potential for applications in microdosimetry in clinical beam conditions.