We present a label-free biomarker detection method based on immunoaggregation and resistive pulse sensing technology. In this approach, target biomarkers and antibody (Ab)-functionalized microparticles are mixed to form biomarker–microparticle aggregates. A resistive pulse sensor is then used to measure the sizes and count the number of aggregates. The measured volume fraction of the aggregates represents the concentration of the targeted biomarker. In our tests, human ferritin, used as a biomarker, triggered the aggregation of antiferritin Ab-functionalized microparticles in phosphate-buffered saline (PBS). The volume fraction of aggregates increased with the increased ferritin concentration. We also demonstrated the detection of human ferritin in 10% fetal bovine serum (FBS) to mimic a real detection environment in complex media. The detection range from 0.1 to 208 ng/mL was achieved. In addition, we demonstrated that the detection range can be shifted to lower and higher concentrations by decreasing and increasing microparticle concentrations. This biomarker detection method is label-free, rapid, and able to quantitatively measure the concentration of any macromolecular biomarker as long as an antibody can be found, with simple measurement setup and sample preparations.