The beta-secretase BACE1 is a central drug target for Alzheimer’s disease. Clinically tested, BACE1-directed inhibitors also block the homologous protease BACE2. Yet, little is known about physiological BACE2 substrates and functions in vivo. Here, we performed discovery proteomics to identify substrates of the protease BACE2 in plasma of mice. Therefore, we analysed plasma from BACE2 KO, and WT controls. Inactivation of BACE2 inhibited shedding of VEGFR3/FLT4. Thus, sVEGFR3 represents a pharmacodynamic plasma marker for BACE2 activity in vivo.