ABSTRACTSynaptic degeneration has been linked to cognitive decline. The presynaptic protein, synaptosomal‐associated protein 25 kDA (SNAP‐25), is crucial for synaptic transmission and has been suggested as a biomarker in Alzheimer's disease (AD). In the current study, we investigated the ability of SNAP‐25 to differentiate between heterogenous dementia etiologies and whether SNAP‐25 could be a staging marker in AD. SNAP‐25 in the cerebrospinal fluid (CSF) from a retrospective (n = 187) and a prospective (n = 134) cohort was investigated with immunoprecipitation mass spectrometry (IP‐MS) and single‐molecule array (Simoa), respectively. Both cohorts consisted of healthy controls (HC) and patients with cognitive decline of different etiologies. CSF SNAP‐25 concentration was higher in AD and non‐neurodegenerative diseases (i.e., vascular dementia) compared with controls but did not differ between AD and non‐AD neurodegenerative diseases. We found a trend toward an association between SNAP‐25 and disease burden when comparing HC, mild cognitive impairment due to AD, and AD. CSF SNAP‐25 concentrations were strongly associated with CSF phosphorylated tau (p‐tau) concentrations, thus strengthening the link between synaptic dysfunction and tau pathophysiology in AD. Our initial findings suggest that SNAP‐25 may be a potential biomarker for differentiating AD from dementia due to other etiologies. However, due to the significant association between SNAP‐25 and p‐tau proteins, the clinical utility of SNAP‐25 as a diagnostic biomarker for AD may be limited, while SNAP‐25 may be useful for monitoring disease progression or treatment response.