Background Recent evidence suggests that a high refined‐carbohydrate diet is a risk factor for dementia, especially among APOE4 carriers. Thus, refined‐carbohydrate diet may modify biomarkers of dementia, such as amyloid‐β (Aβ) peptides according to APOE4 carrier status. Here, we focus on afternoon‐snack glycemic load (GL) because snacks are generally richer in refined carbohydrates. We investigated the relationship between plasma Aβ, afternoon‐snack GL, and APOE4 carrier status in participants with or without incident dementia. Method We followed 377 non‐demented participants from the Three‐City Study (Bordeaux and Montpellier center), aged over 65, with plasma Aβ peptide concentrations measured at baseline. At 2‐year (Bordeaux center) and 4‐year (Montpellier center), the participants completed a semi‐quantitative Food Frequency Questionnaire which was used to estimate GL (GL assesses both the carbohydrate quantity and quality, and reflects glycemic response). At each follow‐up (every 2‐3 years during 15 years), participants had clinical diagnosis of dementia. We used linear regressions to evaluate the associations between plasma Aβ40, Aβ42, Aβ42/Aβ40 ratio and GL, APOE4 carrier status, and the interaction GL x APOE4, among participants with or without incident dementia. Models were adjusted for center, age, sex, education level, total cholesterol, serum creatinine and adherence to the Mediterranean diet. Result The sample included 60.2% of women and mean age of the participants was 76.1 (± 5.2) years. During the follow‐up, 51 participants developed dementia. Among dementia‐free participants, GL tended to be associated with Aβ42/Aβ40 ratio (β = ‐0.006 SE = 0.003, P = .055) but not with Aβ40 and Aβ42 concentrations. Among incident dementia participants, the interaction GL x APOE4 was associated with plasma Aβ40 concentration (β = 54.2 SE = 25.6 pg/mL, P = .042). Regarding Aβ42, no interaction was found. Regarding Aβ42/Aβ40 ratio, GL x APOE4 was no longer significant after adjustment (β = ‐0.038 SE = 0.024 P = .124). Conclusion In APOE4 carriers with incident dementia, a higher afternoon‐snack GL was associated with worse plasma Aβ peptide concentrations. These results highlight that rich refined‐carbohydrate diet is a modifiable risk factor for plasma Aβ peptide concentrations and subsequent dementia risk. Experimental studies are required to explain potential mechanism involved.