Alzheimer's disease (AD) is an irreversible neurodegenerative disorder. Currently, there are no available treatments that can effectively stop or reverse the progression of the disease, and existing therapeutics can only alleviate the symptoms. Thus, it remains urgent to develop effective early-stage AD diagnostic methods. In recent years, the search for near-infrared fluorescent (NIRF) probes of AD hallmarks has become a promising research field. In this Review, we will focus on small-molecule NIRF probes used to detect β-amyloid, tau proteins, and reactive oxygen species in vivo during the past 4 years. We believe that some new directions we raise herein will benefit the future development of NIRF probes in the field of AD research.