Background Subjective cognitive decline, a risk factor of Alzheimer’s disease, was previously associated with subtle alterations in the hippocampus, a potential target for early intervention. This proof-of-concept human trial evaluated impacts of a novel dietary spermidine supplementation on hippocampus-dependent memory functions in individuals at increased risk for Alzheimer’s disease. Methods Healthy older participants with subjective cognitive decline and self-reported worry (n = 30, mean age: 70 ± 5 years, 2 drop-outs) were included in the 3-months randomized, placebo-controlled, double-blind trial. Episodic memory was assessed using a behavioral paradigm known to involve the hippocampal formation. Behavioral performance, measured using a recognition memory and a pattern separation score, was compared between intervention groups (polyamine and placebo) at the end of intervention, co-varying for baseline levels. Results Pattern separation, but not recognition memory, was enhanced in the spermidine compared to placebo-treated group at the end of intervention with a medium effect size (Cohen’s d = 0.74). Within group comparisons indicated improvement of pattern separation performance in the spermidine-treated (Cohen’s d = 0.80) and not in the placebo-treated (Cohen’s d = 0.20) group. A similar effect was not observed for recognition memory performance. Discussion Spermidine supplementation may induce positive effects in pattern separation, an early behavioral marker of memory changes associated with Alzheimer’s dementia. This finding allows for longer-term intervention studies in humans to investigate impacts of oral spermidine intake on episodic memory and hippocampal integrity. The spermidine-rich plan extract could perhaps increase reserve mechanisms in older individuals at risk for Alzheimer’s disease.