ABSTRACT. Psychobiotics are promising modulators of the Microbiota-Gut-Brain axis with potential benefits to well-being and mental health. However, their effectiveness still needs to be further addressed. This randomized placebo-controlled clinical trial (NCT04823533) aimed to evaluate the effects of a probiotic formulation (Lactobacillus helveticus R0052 and Bifidobacterium longum R0175) on psychological processes. 135 healthy adults were randomized to 4 weeks of probiotic/placebo treatment. Primary outcome was the Ryff well-being scale. Secondary outcomes were the Satisfaction with Life Scale, the Positive and Negative Affect Scale, the Short Form Health Survey, the Difficulties in Emotion Regulation Scale, and the State-Trait Anxiety Inventory. Exploratory outcomes were the Five Facet Mindfulness Questionnaire and the Multidimensional Assessment of Interoceptive Awareness. Lifestyle behaviors were assessed using a custom-made scale. Data was collected at baseline, post-intervention and follow up (after 8 weeks). Repeated Measures ANOVA was conducted for overall analysis. For exploratory analyses, we clustered three lifestyle behaviors categories: Healthy Behaviors, Risk behaviors, and Undetermined Behaviors. Correlations were calculated between each lifestyle category and each scale's mean scores. Finally, Linear-Mixed effect models were used to analyze the interaction between treatment and lifestyle categories. Results showed no significant effects in whole sample outcomes. Exploratory results showed Healthy Behaviors were significantly correlated with most psychological measures. Interestingly, the interaction between high scores in Healthy Behaviors and probiotic intake was the single significant predictor of positive effects on anxiety, difficulties in emotion regulation, and mindfulness in the post-treatment outcomes. Conclusions highlight the relevance of controlling for lifestyle behaviors in psychobiotic and mental health research.