Background:: PYX-106 is a novel monoclonal antibody (mAb), targeting the sialic acidbinding immunoglobulin-like lectin 15 (Siglec-15) in the Tumor Microenvironment (TME). Precise measurement of PYX-106 is essential for the thorough assessment of PYX-106 pharmacokinetics in clinical investigations. Methods:: A novel Electrochemiluminescence (ECL) immunoassay for the quantitation of PYX- 106 in human serum was developed and validated. Biotinylated anti-PYX-106 antibody Bio-A1A1 was employed as the capture antibody, and ruthenylated anti-PYX-106 antibody Ru-A3G10 was utilized as the detection antibody in the ECL immunoassay on Meso Scale Discovery (MSD) platform. Results:: This assay was fully validated in terms of selectivity, accuracy, precision, hook effect, stability, etc., with a dynamic range from 50.0 to 2,500 ng/mL in human serum under the 2018 U.S. Food and Drug Administration (FDA) guidance and the 2022 U.S. FDA ICH M10 guidance. Conclusion:: PYX-106 bioanalytical assay validation was reported for the first time in a biological matrix, and this assay has been successfully applied to support a clinical trial PYX-106-101.