Probiotics are an effective adjuvant treatment for Alzheimer's disease (AD) through modulating the gut-brain axis. We aimed to evaluate the effects of two single-strain probiotics on cognition, physical activity, and anxiety in subjects with mild-to-moderate AD. Patients (n = 90) were randomly assigned to Lactobacillus rhamnosus HA-114, Bifidobacterium longum R0175 (both 10¹⁵ CFU twice daily), or placebo for 12 weeks. Primary outcomes were the Mini-Mental State Examination (MMSE) and categorical verbal fluency test (CFT). Secondary outcomes included Activities of Daily Living (ADL), Lawton Instrumental ADL (IADL), and Generalized Anxiety Disorder (GAD-7) scales. Linear mixed-effect models were used for analyses. Both probiotic groups improved significantly versus placebo on MMSE, with B. longum demonstrating greater cognitive improvement (B. longum: difference 4.86, 95% CI: 3.91–5.81; L. rhamnosus: difference 4.06, 95% CI: 3.11–5.01; both P < 0.0001). CFT improved significantly in both groups (B. longum: 3.90, 95% CI: 3.22–4.57; L. rhamnosus: 3.46, 95% CI: 2.79–4.13; both P < 0.0001). IADL improved versus placebo (B. longum: 1.50, 95% CI: 0.93–2.06; L. rhamnosus: 1.05, 95% CI: 0.49–1.61; both P < 0.0001), while ADL showed no significant change (P = 0.24). GAD-7 was significantly reduced in both groups (both P < 0.0001). Single-strain design enabled strain-specific attribution, avoiding inter-strain competition that may limit multi-strain formulations. These findings support targeted probiotic supplementation as a safe, cost-effective adjuvant strategy in aged care settings to improve cognition, daily functioning, and anxiety in AD, with potential to slow functional decline and reduce caregiver burden.