Spider mites are important agricultural pests worldwide. Recent advances in multi-omics approaches have overcome the hurdle posed by their small body size, accelerating our understanding of their biology, particularly regarding host-plant interactions and the evolution of acaricide resistance. However, studies on their symbiotic bacteria have been largely focused on Wolbachiaand Cardinium, leaving their bacterial microbiome poorly characterized. Interactions between spider mites and their bacterial microbiome may harbor novel mechanisms underlying xenobiotic metabolism and feeding behavior, which could contribute to the development of alternative control technologies. This study aimed to elucidate the bacterial microbiome throughout the life stages of the two-spotted spider mite (TSSM) Tetranychus urticae and the Kanzawa spider mite (KSM) T. kanzawai and to identify their associated bacteria. Both species were reared on kidney bean leaves. Assuming essential bacteria are present throughout all developmental stages, we examined the entire life cycle. To identify bacteria acquired through feeding, we collected samples before and after feeding initiation. We analyzed eight developmental groups: eggs, pre- and post-feeding larvae, protonymphs, deutonymphs, newly emerged (0-day-old) adults, 1-day-old adults, and 5-day-old adults. To minimize contamination, the surfaces of eggs were washed with 0.1% Tween 20 using a 60-µm mesh. After washing, removal of surface debris including bacteria was confirmed by scanning electron microscopy. DNA was extracted and the V3–V4 region of the 16S rRNA gene was sequenced. SYTO9-labeled bacteria were orally administered to TSSM to verify environmental bacterial acquisition. Microbiome analysis revealed that Wolbachia dominated both species across all stages (75–99%), confirming its status as the primary symbiotic bacterium. Eggs and pre-feeding larvae harbored additional bacterial genera, suggesting transgenerational transmission of symbiotic bacteria. We identified bacteria that were absent from eggs and pre-feeding larvae, but present post-feeding: five genera in TSSM and six in KSM, with two genera shared by both, likely acquired through feeding. Fluorescence microscopy confirmed TSSM bacterial uptake and survival, providing the first demonstration of environmental bacterial acquisition in spider mites. Future studies will investigate bacterial abundance, localization, viability, and host-microbe interactions to develop novel control strategies targeting these bacteria.