This study aimed to investigate whether dietary supplementation with heat-inactivated Limosilactobacillus reuteri PSC102 (LR) enhances immunological responses and growth performance in weaned pigs. Twenty-five weaned piglets were randomly allocated to five groups. Each treatment group had separate pens with five pigs each. The pigs were fed a diet supplemented with heat-inactivated LR at a concentration of 1×10⁹ CFU/g for 28 days. Throughout the study period, regular measurements, including body weight, food consumption rate, and diarrhoeal frequency, were conducted. At the end of the experiment, blood samples were collected to assess cytokine and haptoglobin levels. Faecal samples, rectal swabs, and nasal swabs were collected for analysis of pathogenic bacterial growth using selective media. Additionally, faecal samples were analysed to determine the composition of the microbiota. Heat-inactivated LR enhances growth rate and feed efficiency while decreasing diarrheal frequency in weaned piglets. Compared to pigs fed a control diet, diets containing heat-inactivated LR exhibited increased serum levels of cytokines and decreased haptoglobin levels, demonstrating a beneficial effect on the presence of pathogenic microorganisms. Heat-inactivated LR diets alter the microbiota of piglets by enhancing alpha diversity and increasing the composition of beneficial bacteria. Overall, dietary heat-inactivated LR enhanced growth rates, regulated immunological responses in weaned pigs, and reduced post-weaning diarrhoea. Therefore, this suggests its potential as a health functional ingredient for humans and animals.