Autoantibodies raised against tumor-associated antigens have shown high promise as clinical biomarkers for reliable diagnosis, prognosis, and therapy monitoring of cancer. An electrochemical disposable biosensor for the specific and sensitive determination of p53-specific autoantibodies has been developed for the first time in this work. This biosensor involves the use of magnetic microcarriers (MBs) modified with covalently immobilized HaloTag fusion p53 protein as solid supports for the selective capture of specific autoantibodies. After magnetic capture of the modified MBs onto screen-printed carbon working electrodes, the amperometric signal using the system hydroquinone/H2O2 was related to the levels of p53-autoantibodies in the sample. The biosensor was applied for the analysis of sera from 24 patients with high-risk of developing colorectal cancer and 6 from patients already diagnosed with colorectal (4) and ovarian (2) cancer. The developed biosensor was able to determine p53 autoantibodies with a sensitivity higher than that of a commercial standard ELISA using a just-in-time produced protein in a simpler protocol with less sample volume and easily miniaturized and cost-effective instrumentation.