This paper describes the development of a low-cost, modular system for the rheological characterization of ferrofluids under the influence of magnetic fields. The proposed system is compatible with conventional rotational rheometers that are fabricated using 3D printing techniques. The design encompasses a primary component that can be adapted to varying magnetic field strengths through the integration of neodymium magnets. Additionally, the design incorporates a calibrator that facilitates the vectorial characterization of the magnetic field by using a Cartesian coordinate system. The system enables precise alignment between the applied field and the measurement geometry in the rotational rheometer, and it supports tests at elevated temperatures with natural cooling. Experimental validation was performed using a magnetite ferrofluid in 0.8% v/v ethylene glycol, and the rheological behavior was evaluated under different magnetic field strengths.