Feline mammary carcinoma (FMC) is a common cancer found in female cats. Early detection is necessary to prevent both local and distant metastasis and increase survival times. However, the available protein biomarkers involved in non-metastatic FMC (NmFMC) and metastatic FMC (mFMC) are limited. The purpose of this study was to identify serum peptidome profiles in NmFMC and mFMC using liquid chromatography-tandem mass spectrometry.