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Alzheimer’s disease (AD) is the leading cause of dementia, affecting over 50 million people worldwide, with prevalence expected to triple by 2050. Early detection is crucial for clinical trial enrollment and care planning, and multimodal data (MRI, PET, CSF biomarkers, and cognitive assessments) provides complementary information on neurodegeneration, metabolism, and protein aggregation. This systematic review synthesizes AI/ML approaches for early AD detection using multimodal data, focusing on fusion strategies and performance across disease stages. Following PRISMA guidelines, searches of PubMed, IEEE Xplore, Scopus, Web of Science, and arXiv (2017–2023) identified studies using ML/DL with at least two modalities and reporting diagnostic performance. From 1,247 records, 35 studies were included. MRI was the most used modality (>90%), followed by cognitive tests (70–80%), PET (40–50%), and CSF (20–30%). Early fusion was most common, with increasing use of intermediate fusion. Multimodal models achieved AUROC of 0.90–0.98 for AD vs controls, but lower performance (0.70–0.85) for predicting MCI conversion to AD. Overall, multimodal AI improves early AD detection, with strong performance for diagnosis but persistent challenges in forecasting MCI progression due to heterogeneity and limited longitudinal data.