The development of a sensitive, specific, and non-invasive approach for cancer detection will facilitate early detection and, hence, improve the outcome of individuals with known cancer predispositions. Proteomic profiling of blood emerges to be a logical choice of such non-invasive or minimal invasive detection. However, plasma biomarker discovery of pediatric cancers lags behind that of adult cancers, suggesting more efforts are needed in this area. In this study, we used surface-enhanced laser desorption/ionization-time of flight mass spectrometry to profile plasma proteome in osteosarcoma patients. Osteosarcoma is a bone cancer that affects many children and young adults. We have shown that the plasma proteome contains a unique cancer signature that can distinguish patients with osteosarcoma from those with a benign bone disease. To improve cancer biomarker discovery in plasma, we have also shown that depletion of two highly abundant plasma proteins increases the detection sensitivity of lower-abundance proteins. The combination of depletion and proteomic profiling may increase the chance of identifying tumor-derived proteins within the plasma of pediatric cancer patients.