The ideal plant architecture is a critical factor in achieving high yields in tomato (Solanum lycopersicum) cultivation.  The length and number of internodes directly influence plant height.  Therefore, investigating the regulatory mechanisms of internode morphology is essential for the genetic enhancement of tomatoes.  We identified a naturally occurring field mutant, tomato elongated internode (tei), characterized by longer internodes and darker leaf color.  Physiological hormone and microscopic studies revealed that, compared to wild-type (WT) plants, the tei mutant exhibited increased endogenous GA3 levels, enhanced photosynthetic capacity, and elongation of stem internode cells.  RNA-seq analysis results of tei and WT indicated enrichment in the gibberellin pathway.  We employed BSA-seq for mapping analysis on tei, WT, and F₂ populations, leading to the fine mapping of the candidate gene designated as TEI (Tomato Elongated Internode).  This gene encoded a gibberellin 20 oxidase (GA20ox) protein and was identified as Solyc09g042210.  Additionally, we discovered numerous SNPs and InDel mutations in the TEI promoter region, with expression levels of TEI in tei stems significantly higher than those in WT.  Furthermore, knocking out the TEI gene eliminated its role in elongating internodes.  We proposed that TEI serves as the primary effector gene regulating the internode elongation phenotype associated with tei.  This discovery offered researchers a novel target for enhancing crop plant varieties by modulating gibberellin homeostasis, ultimately contributing to the breeding of superior tomato varieties.