The adhesion molecule CD99 is essential for transendothelial migration (TEM) of leukocytes. Here we demonstrate by biochemical and cellular assays that CD99 undergoes ectodomain shedding by the metalloprotease meprin β and subsequent intramembrane proteolysis by γ-secretase. The cleavage site in CD99 was identified by mass spectrometry within an acidic region highly conserved through different vertebrate species.