Synaptic degeneration is the pathologic foundation of cognitive decline in the Alzheimer's disease (AD) continuum. We aimed to determine whether cerebrospinal fluid (CSF) synaptic marker neurogranin (Ng) is a disease state or a disease stage biomarker in the AD continuum. Studies comparing CSF Ng levels among AD, mild cognitive impairment (MCI) and healthy participants were included. Studies were eligible if the correlation between CSF Ng levels and Mini-Mental Status Examination (MMSE) scores was investigated. Twenty-one studies met our inclusion criteria (n = 4515). The magnitude of effect sizes was more apparent in AD (standardized mean difference [SMD] = 1.72; 95% confidence interval [CI] = 1.23-2.22), than in MCI (SMD =0.82; 95% CI =0.29-1.34) compared to control populations. These results suggest that CSF Ng can discriminate AD and MCI from control populations, implying that synaptic degeneration worsens as patients progress from MCI to AD. However, there was a very weak correlation between CSF Ng levels and MMSE scores (r = -0.15; 95% CI = -0.21--0.08) among the whole populations, suggesting that an increment of CSF Ng is best considered a biological evidence of disease state in the AD continuum. Our study provides evidence that the synaptic marker CSF Ng can be used as a disease state biomarker for the AD continuum. Because synaptic degeneration is a distinct pathologic event from amyloid deposition and neurofibrillary tangle formation, CSF Ng may provide an important supplementation to the AT(N) biomarker system to reveal the sequence of neuropathology.