Background/Objectives: Soy foods and isoflavones are inversely associated with cognitive decline; however, randomized controlled trials (RCTs) show mixed results. Fermented soy contains bioactive compounds not found in unfermented soybeans, such as peptides and aglycone isoflavones, which may support cognition by reducing neuroinflammation and oxidative stress. Fermented soy RCTs on older adults with mild cognitive impairment show cognitive benefits; however, the effects of fermented soy on cognitively healthy older adults are not known. Methods: We investigated the effects of a non-probiotic fermented soy powder, added to the usual diet, compared to a placebo matched for energy on global cognition, memory, verbal fluency, processing speed, and executive function in a 12-week RCT on 61 adults aged 65 years and older (74 ± 5 y; 47 completers). Results: The fermented soy group showed significant improvement (p = 0.041) in memory scores (1.81%, 95% CI: −2.10, 5.72) vs. placebo (1.16%, 95% CI: −3.64, 5.97) using analysis of variance, adjusted for sex and baseline scores. Post hoc analyses on women 70 years and older (n = 29) found significant improvement in global cognition (p = 0.028) and memory (p = 0.049) in the fermented soy group. Global cognition mean change adjusted for baseline scores was 2.86% (95% CI: 1.52, 4.21) for fermented soy and 0.06% (95% CI: −1.43, 1.55) for placebo. Memory mean change adjusted for baseline scores was 8.47% (95% CI: 5.05, 11.89) in the fermented soy group, compared to 2.05% (95% CI: −1.75, 5.84) for placebo. Conclusions: These outcomes suggest that fermented soy has the potential to slow age-related cognitive decline, especially memory for women 70 and older. Further research to confirm these findings in older women, and in males and other age categories is warranted.