Brain aging is characterized by memory loss and cognitive impairment. With the growth of the population and advances in medical care, the size of the aging population is increasing. Therefore, the discovery of anti-aging drugs has become a popular topic in recent years. Fibroblast growth factor 21 (FGF21) has been reported to inhibit oxidative stress, reduce inflammation, and delay senescence. The present study was designed to investigate the effects of recombinant human FGF21 (rhFGF21) on senescence in the brain in a mouse model of D-galactose (D-gal)-induced aging. The behavioral tests revealed that rhFGF21 improved D-gal-induced learning and memory impairment in mice. RhFGF21 improved the morphology of cortical and hippocampal neurons and increased the expression of PSD95 in the model mice. RhFGF21 reduced the number of microglia and astrocytes in the cortex and hippocampus, increased the activities of the antioxidant enzymes (GSH-PX, CAT, and SOD), and inhibited the expression of p-NFκB and p53 proteins, as well as the mRNA expression of the inflammatory cytokines (IL-1β, IL-6, TNFα, and iNOS). SIRT1 regulates senescence and inflammation, and FGF21 participates in physiological and pathological processes by binding to the FGFR1. Therefore, we measured SIRT1 and activated FGFR1 (p-FGFR1) levels. RhFGF21 administration increased the expression of cortical and hippocampal SIRT1 and p-FGFR1 in D-gal-induced aging mice. These data suggested that rhFGF21 alleviated learning and memory impairment in a mouse model of D-gal-induced aging by increasing antioxidant enzyme activity, inhibiting inflammation, and senescence-related gene expression via modulating FGFR1 and SIRT1.