INTRODUCTION: Glial fibrillary acidic protein (GFAP) may contribute to Alzheimer's pathology at early disease stages. GFAP moderation of Alzheimer's disease (AD)-related neurodegeneration and cognition is unclear. METHODS: We examined plasma GFAP moderation of AD biomarkers (amyloid beta [Aβ]-positron emission tomography [PET][A]; plasma phosphorylated tau-181 [p-tau181][T]), neurodegeneration (plasma NfL[N]; structural magnetic resonance imaging [MRI][N]), and cognition (Cog; Cog) in two cohorts: University of California San Francisco (UCSF) (N = 212, 91.0% non-Hispanic/Latino White [NHLW], age = 74.7 [7.6] years, 75.9% cognitively unimpaired [CU]) and 1Florida Alzheimer's Disease Research Centers (1FLADRC; N = 582, 32.8% NHLW, age = 70.7 [8.5] years, 28.9% CU). RESULTS: Plasma GFAP consistently moderated A-T (UCSF: β = 0.46, p = 0.012; 1FLADRC: β = 0.12, p = 0.029). The association between elevated Aβ-PET and increased (p-tau) was strengthened at higher GFAP concentrations. In 1FLADRC, GFAP moderated T-N In UCSF, GFAP moderated T-Cog and N-Cog. Higher GFAP consistently related to worse neurodegeneration and cognition (main effects). DISCUSSION: Across demographically and clinically heterogeneous cohorts, plasma GFAP is a key moderator of AD and may help identify individuals at greatest risk of AD-related neurodegeneration and cognitive decline. HIGHLIGHTS: AD biomarkers were measured in two demographically and clinically distinct cohorts. Plasma GFAP moderated Aβ-PET to p-tau associations in both UCSF and 1FLADRC. Cohort-dependent, GFAP moderated p-tau to neurodegeneration and cognition associations. All moderations revealed strengthened disease associations with higher plasma GFAP. Plasma GFAP may help identify individuals at greatest risk of AD-related decline.