This article presents a novel sensor topology employing an interdigital capacitor (IDC) integrated into a half-mode substrate-integrated waveguide (HMSIW) for permittivity measurement of solid materials. HMSIW with around 50% smaller size compared with SIW offers a compact solution for hosting the sensing section. Capacitive-like characteristic of IDC further improves the size reduction by up to 71.6%. In addition, IDC with enhancing the electric field concentration is exploited for the sensing section. Design details of the hosting section and the sensing section are provided along with extraction and analysis of the equivalent circuit model of the sensor. Design and simulations are carried out by means of design tools, and the proposed sensor is fabricated on a 50 x 24 x 1.6 mm(3) printed circuit board (PCB). Measurements demonstrate that the fabricated sensor prototype shows 113-MHz frequency shift per unit permittivity change around 3.5 GHz, and it has a normalized sensitivity of 3.16% within a wide range of permittivity values, which is relatively better than similar sensors in the literature.