Molecular hydrogen (H2) is a versatile therapeutic agent. H2gas inhalation is reportedly safe and has a positive impact on a range of illnesses, including Alzheimer’s disease (AD). Herein, we investigated the effects of 4 weeks of H2gas inhalation on community-dwelling adults of various ages. Fifty-four participants, including those who dropped out (5%), were screened and enrolled. The selected participants were treated as a single group without randomization. We evaluated the association between total and differential white blood cell (WBC) counts and AD risk at individual levels after 4 weeks of H2gas inhalation treatment. The total and differential WBC counts were not adversely affected after H2gas inhalation, indicating that it was safe and well tolerated. Investigation of oxidative stress markers such as reactive oxygen species and nitric oxide showed that their levels decreased post-treatment. Furthermore, evaluation of dementia-related biomarkers, such as beta-site APP cleaving enzyme 1 (BACE-1), amyloid beta (Aβ), brain-derived neurotrophic factor (BDNF), vascular endothelial growth factor A (VEGF-A), T-tau, monocyte chemotactic protein-1 (MCP-1), and inflammatory cytokines (interleukin-6), showed that their cognitive condition significantly improved after treatment, in most cases. Collectively, our results indicate that H2gas inhalation may be a good candidate for improving AD with cognitive dysfunction in community-dwelling adults of different ages.