The usage of probiotics has expanded beyond the areas of gut and immune health improvement. Several studies have shown the positive impact associated between probiotics and stress, cognition, and mood; a relationship referred to as the gut–brain axis. Method: The aim of this exploratory study was to evaluate the effect of the probiotic strain Lactiplantibacillus plantarum HEAL9 (LPHEAL9) on the gut–brain axis in subjects with moderate stress. One hundred and twenty-nine subjects aged 21–52 years completed the study, randomized to consume either LPHEAL9 or placebo for 12 weeks. Results: Perceived stress and awakening cortisol were significantly reduced over time in both groups. A significant improvement in four cognition tests after consumption of LPHEAL9 compared to placebo was observed (rapid information processing test, numeric working memory test, paired associated learning, and word recall. There was a tendency for a significantly better improvement in the LPHEAL9 group for three mood subscales (Confusion–Bewilderment, Anger–Hostility, and Depression–Dejection) and for fewer subjects with poor sleep in the LPHEAL9 group compared to placebo. Conclusions: Intake of LPHEAL9 significantly improved cognitive functions compared to the placebo, potentially by ameliorating aspects of mood and sleep.