This paper presents an efficient yet straightforward passive reconfiguration technique to tune the operating frequency of a branch-line crossover (BLCO). The underlying principle is to fill rectangular dielectric channels (RDCs) prepared beforehand with either air or materials of different relative permittivity. Two configurations (one RDC and three RDCs in each arm) of the branch-line crossover are employed to estimate the tunability range of the operating frequency. The introduction of RDCs packed with different materials in the branch lines modifies the effective permittivity of the dielectric medium, resulting in an alteration of the operating frequency. The size and the positions of the RDCs are optimized using full-wave electromagnetic simulations to achieve maximum tunability range while ensuring reasonable bandwidth. A lumped circuit model (LCM) is developed to analyze the working principle of the proposed technique. To validate computational models, two prototypes of the branch-line crossover are realized, fabricated, and experimentally demonstrated. The first BLCO packed with seven RDCs exhibits a frequency tuning range of 15.8%, whereas the second prototype filled with twenty-one RDCs features a tuning range of 36.9%. Branch-line crossover, reconfigurable, dielectric-loaded. INDEX TERMS