<i>Background:</i> The serotonin 5-HT₄ receptor is of potential interest for the treatment of Alzheimer’s disease because it increases memory and learning and influences amyloid precursor protein (APP) processing. Several 5-HT₄ receptor isoforms have been cloned in humans. They all belong to the G-protein-coupled receptor superfamily and differ in composition and length of their amino acid intracellular C-terminal sequence. At present, it is still unknown whether 5-HT₄ receptor isoforms may have distinct effects on APP processing. <i>Objective:</i> In this study, we investigated the effect of an ultrashort isoform of the human 5-HT₄ receptor (2 amino acids after the splicing site), the 5-HT_4(d) receptor isoform (h5-HT_4(d)), on APP processing in Chinese hamster ovary cells (CHO cells). <i>Methods:</i> Non-amyloidogenic soluble sAPPα was determined by immunoblot and β-amyloid peptides (Aβ_1–40 and Aβ_1–42) were assayed by ELISA in CHO cells stably expressing h5-HT_4(d) and transiently transfected with human APP₆₉₅. <i>Results:</i> We show here that activation of h5-HT_4(d) strongly stimulates the release of sAPPα and concomitantly decreases extracellular Aβ peptides. These data contrast with our previous findings showing that the h5-HT_4(e/g) receptor isoform, which has 20 amino acids after the splicing site, did not influence Aβ secretion. <i>Conclusion:</i> We conclude that C-terminal tails of 5-HT₄ receptor isoforms may influence their functional effect on Aβ secretion and that the pathways regulating either β- or γ-secretase may be differentially controlled by 5-HT₄ receptor isoforms.