Most hereditary forms of hemophagocytic lymphohistiocytosis (HLH) are caused by defects of cytotoxicity, including the vesicle trafficking disorder Griscelli syndrome 2 (GS2, RAB27A deficiency). Deficiency of the mitogen activated protein (MAP) kinase activating death domain protein (MADD) results in a protean syndrome with neurological and endocrinological involvement. MADD acts as a guanine-nucleotide exchange factor for small GTPases, including RAB27A. A homozygous splice site mutation in MADD was identified in a female infant with syndromal features, secretory diarrhea, and features of HLH. Aberrant splicing caused by this mutation leads to an in-frame deletion of 30 bp and favors other aberrant variants. Patient NK cells and cytotoxic T cells showed a severe degranulation defect leading to absent perforin-mediated cytotoxicity. Platelets displayed defective ATP secretion, comparable to GS2. To prove causality, we introduced a CRISPR/Cas9 based MADD knock-out in the NK cell line NK-92mi. MADD deficient NK-92mi cells showed a degranulation defect and impaired cytotoxicity similar to that of the patient. The defect of cytotoxicity was confirmed in another MADD deficient patient. In conclusion, RAB27A-interacting MADD is involved in vesicle release by cytotoxic cells and platelets. MADD deficiency causes a degranulation defect and represents a novel disease predisposing to an HLH phenotype.