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Accumulation of neurotoxic βamyloid (Aβ) is a major hallmark of Alzheimer's disease (AD)1. Formation of Aβ is catalyzed by γsecretase, a protease with numerous substrates2,3. Little is known about the molecular mechanisms that confer substrate specificity on this potentially promiscuous enzyme. Knowledge of the mechanisms underlying its selectivity is critical for the development of clinically effective γ-secretase inhibitors that can reduce Aβ formation without impairing cleavage of other γ-secretase substrates, especially Notch, which is essential for normal biological functions3,4. Here we report the discovery of a novel γ-secretase activating protein (gSAP), which dramatically and selectively increases Aβ production through a mechanism involving its interactions with both γsecretase and its substrate, the amyloid precursor protein C-terminal fragment (APP-CTF). gSAP does not interact with Notch nor does it affect its cleavage. Recombinant gSAP stimulates Aβ production in vitro. Reducing gSAP levels in cell lines decreases Aβ levels. Knockdown of gSAP in a mouse model of Alzheimers disease reduces levels of Aβ and plaque development. gSAP represents a new type of γ-secretase regulator that directs enzyme specificity by interacting with a specific substrate. We demonstrate that imatinib, an anti-cancer drug previously found to inhibit Aβ formation without affecting Notch cleavage5, achieves its Aβ-lowering effect by preventing gSAP interaction with the γ-secretase substrate, APP-CTF. Thus, gSAP can serve as an Aβ-lowering therapeutic target without affecting other key functions of γ-secretase.