Increasing recognition of the interaction between the brain and gut microbiota has spurred interest in the efficacy of probiotics on stress-related behaviours. This study aimed to elucidate the effects of Lactobacillus gasseri CP2305 (CP2305) on mental and physical states in healthy male students enrolled in a cadaver dissection course in a double-blinded, placebo-controlled, crossover-designed trial. Daily administration of CP2305 for 4 weeks significantly improved anxiety, depressive mood and global sleep quality compared with placebo. CP2305 also suppressed salivary cortisol release. In the faecal microbiota, CP2305 suppressed the growth of Enterobacteriaceae. Differential changes in gene expression in peripheral blood leukocytes during the administration period were observed between the placebo and CP2305 groups. Eukaryotic initiation factor 2-related genes were down-regulated only in the placebo group. Thus, the ingestion of CP2305 may have beneficial effects on stress-associated behaviours.