Detection of biomarkers at low concentrations is essential for early diagnosis of numerous diseases. In many sensitive assays, the target molecules are tagged using fluorescently labeled probes and captured using magnetic beads. Current devices rely on quantifying the target molecules by detecting the fluorescent signal from individual beads. Here, we propose a compact fluorescence-based magnetically aggregated biosensors (MAB) system. Using the device to detect human Interleukin-8, we demonstrated a 0.1 ng/L limit of detection and a 4-log dynamic range, performance which is on par with the most sensitive devices, but is achieved without their bulk and cost.