The gut bacteria composition in phytophagous insects is closely linked to their developmental stages and can fulfill multiple functions for the host insect. However, few studies have investigated the relationship between gut bacteria and developmental stages in the tomato leaf miner moth, Tuta absoluta (Meyrick). This study aimed to characterize the diversity of gut bacteria in larvae of different instar stages of T. absoluta and to predict the functional roles of these bacteria. We extracted genomic DNA from the gut bacteria of T. absoluta larvae at the 1^st, 2^nd, 3^rd, and 4^th instar stages, and analyzed the diversity and abundance of the gut bacteria using Illumina MiSeq Ⅱ high-throughput sequencing technology. Alpha diversity analysis indicated that gut bacterial diversity and abundance peaked in the 1^st instar larvae of T. absoluta and diminished as instar age increased. The dominant phylum of gut bacteria across all four instar stages was Proteobacteria. Excepting that Enterobacteria was the dominant genus in the 4^th instar larvae, whereas Wolbachia was predominant in the other instars. Predicted functional analysis based on the PICRUSt2 gene database revealed that the gut bacteria in T. absoluta larvae participated in carbohydrate, amino acid, and energy metabolism, suggesting their crucial roles in digestion, absorption, and nutrient provision. Among them, there were significant differences in the metabolism fuction of intestinal bacteria at different instars and the 1^st instar larvae were significantly higher than that in the 3^rd and 4^th instars. This study provides a foundational basis for exploring the interactions between gut bacteria and their host in T. absoluta and offers additional insights and directions towards the adaptive evolution and integrated management of this pest.