The objective of this study is to investigate the complex dielectric properties of watermelon juices using Time-Domain Reflectometry (TDR) over a wide frequency range of 1 GHz to 30 GHz. This analysis provides useful information about the molecular composition and quality indicators. Fresh juice was extracted from watermelon fruit. The juice was then filtered to remove any solid particles before the dielectric measurements were taken. The dielectric properties of the filtered juices were measured at a controlled temperature of 23°C using Time-Domain Reflectometry (TDR). The microwave dielectric spectral analysis of watermelon juice was performed through the measurements, and the analysis of the real, i.e., permittivity, and imaginary dielectric loss components of the complex dielectric constants across the frequency range of 1 GHz to 30 GHz was carried out. The frequency-dependent dielectric constant and loss are presented and discussed. Additionally, a Cole–Cole plot is presented in this study. The findings from this study provide further insights into the molecular polarizability of fruit juice and hence its molecular structure and applicability as a quality indicator in the food industry. This study is the first comprehensive investigation of the complex dielectric properties of watermelon, using TDR across the frequency range from 1 GHz to 30 GHz.