Optimising the production of insects for food and feed and ensuring their health are growing concerns for producers. Insects suffer from a range of insect pathogenic microorganisms, and the management of such diseases is essential. One solution is the introduction of beneficial probiotic bacteria into the diet of the insects. Here, we show that a lactic acid bacterial strain, Pediococcus pentosaceus, isolated from the gut of the mealworm, Tenebrio molitor, was able to inhibit the growth of selected insect pathogens in vitro. Using in vivo assessments of the host’s fitness benefits conferred by the lactic bacterium we show a significant effect of P. pentosaceus on larval growth rate and survival into adulthood. Gut microbiota analysis focusing on bacterial composition based on 16S rRNA gene amplicon sequencing suggests that P. pentosaceus could have successfully colonised the guts, or altered their bacteria, of the larvae that received it. Finally, we discuss our results in the context of mass insect production systems and outline the remaining work needed to explore and secure the role of beneficial bacterial additives in the field.