Lightweight, low-cost, and simple systems for magnetic field sensing are in high demand. Here, we demonstrate such a magnetic field sensing system by integrating a light source, detector, magnetic fluid (MF), and plastic optical fiber (POF). Two bifunctional AlGaInP diodes with identical multiple-quantum well structures separately function as the light source and the detector of the sensing system due to the partial overlap between the electroluminescence and responsivity spectra. Magnetic field sensing is realized by changing the amount of reflected light due to the change in reflection coefficient of the POF/MF interface caused by the ambient magnetic field. The chip-integrated POF magnetic field sensor exhibits a reliable operation with a detection range from 10 Gs to 400 Gs. The results indicate that the chip-integrated POF sensor is promising for magnetic field sensing.