This paper presents a novel, cost-effective printed circuit board (PCB)-integrated microwave sensor with a planar circular complementary split-ring resonator (CSRR) for the dielectric measurement of liquids. The proposed sensor design comprises glass capillary tubes inside the PCB substrate; these tubes contain the samples. Thus, the sensor can estimate the complex permittivity of the sample inserted in the tubes passing through the PCB and parallel to the CSRR sensor plane. These sensors can be manufactured on low-cost substrates, using conventional methods employed in the PCB industry. Subsequently, the sensors can be integrated into the PCB of an electronic circuit. As a proof of principle, a PCB-integrated water–ethanol sensor is fabricated on a FR4 substrate. To evaluate the operation of the sensor, a measurement setup is used to determine the resonant frequency and the Q factor. The complex permittivities of the tested samples are estimated using these determined values. Based on a comparison with the values reported in previous studies, it is concluded that the proposed sensor operates successfully and detects the proportions of each constituent in tested water–ethanol mixtures tested. Moreover, additional types of liquid mixtures can be tested by employing the proposed method.