Xyloglucan endotransglucosylase/hydrolases (XTHs) are key cell-wall-remodeling enzymes that mediate xyloglucan modification, cell expansion, and organ elongation, yet the composition and variation in the XTH gene family across tea accessions remain unclear. In this study, we systematically identified and comparatively analyzed the XTH gene family across the genomes of nine tea accessions, yielding 477 putative XTH genes. Phylogenetic, conserved-motif, protein-domain, and conserved catalytic-motif analyses indicated that the major structural features of tea XTH proteins are broadly conserved, whereas orthogroup-based presence–absence variation (PAV) analysis revealed core, variable, and accession-specific components within the family. Using transcriptomic expression profiles across eight tissues, we selected eight CsXTH genes for internode expression analysis. Reverse transcription quantitative PCR (RT-qPCR) showed that all eight genes were expressed at significantly higher levels in the third internode (I3) than in the second internode (I2) in ‘Longjing 43’ (LJ43), and CsXTH27 retained the same internode-associated expression pattern in ‘Baihaozao’ (BHZ) and ‘Zhongcha 108’ (ZC108), supporting its selection for functional analysis. Transient overexpression of CsXTH27 significantly increased the lengths of I2, I3, and I4, as well as the total length of I1–I4, compared with the empty-vector control. This phenotype was accompanied by significantly increased XTH protein content in stems and an apparent tendency toward larger pith parenchyma cell profiles. Overall, these findings reveal both structural conservation and accession-level variation in the tea XTH gene family and provide preliminary functional evidence linking CsXTH27 to shoot internode elongation.