Dietary interventions have been proposed as an effective way to prevent age-related senescence. Here, we investigated the ability of kale juice (KJ) dietary supplementation to delay cognitive decline in the senescence-accelerated mouse prone 8 (SAMP8) mouse. SAMP8 mice were fed a diet containing 0.8% (w/w) KJ or a control diet for 16 weeks, and cognitive performance was examined using the Morris water maze. KJ administration improved spatial and leaning memory as well as suppressed levels of serum 8-hydroxy-2′-deoxyguanosine and brain malondialdehyde with respect to the control group. Furthermore, the KJ-fed group showed a significant increase in the gene transcription and protein expression of heat shock protein 70 (HSP70). 1-Sinapoyl-2-feruloyl gentiobiose was identified as one of the active components of KJ affecting HSP70 expression. These findings suggest that dietary kale supplementation can suppress cognitive decline and age-related oxidative damage through the activation of HSP70 in SAMP8 mice.