While serology represents the forefront technique for cancer diagnosis, current clinical methods for the detection of serum biomarkers have flaws in terms of the need of complicated manipulations, long analytical time, and high cost. Here, we develop a supramolecular glycoprobe for the quick serological detection of a cancer biomarker. The probe formed by agglutination between self-assembled glyco-gold nanoparticles and a lectin shows subtle optical variations upon the competitive recognition of a glycoprotein biomarker secreted by cancer cells, tumor-bearing mice, as well as clinical cancer patients, with no response to a series of controls including the serum of hepatitis patients. This research provides an insight into the development of effective tools for serological diagnosis of cancer.