Abstract Guar is a legume of industrial importance due to the presence of gum (galactomannan) in its seed endosperm. The mannose (M)/galactose (G) ratio in galactomannan molecule is critical in deciding the physico-chemical properties of guar gum. Gum with reduced galactose content has better commercial value due to better viscosity. We report down regulation of galactomannan galactosyltransferase (GMGT) which links G residues to the M backbone in galactomannan molecule. The target GMGT sequence for RNAi was directionally cloned into pENTRTM/D-TOPO® vector, followed by a LR recombination reaction with pANDA vector. The constructed gmgt RNAi cassette was delivered into guar tissue using Agrobacterium tumefaciens-mediated gene transfer. The cotyledonary node explants were pre-cultured for 72 h and co-cultivated with A. tumefaciens strain harbouring the gmgt RNAi cassette for 24 h. Maximum number of putative transformed shoots regenerated from the cotyledonary node explants cultured in vitro on Murashige and Skoog (MS) medium containing 1.5 mg/L indole-3-acetic acid (IAA), 4.0 mg/L butyric acid (BA) and 1.0 mg/L gibberellic acid (GA3). In vitro regenerated shoots were rooted on half-strength MS medium containing 4% sucrose and 2 mg/L indole-3-butyric acid (IBA). The quantitative Real Time-Polymerase Chain Reaction (qRT-PCR) analysis showed reduced levels of GMGT expression in the seed endosperm of 35 days after flowering (DAF) stage pods. The present findings are expected to be beneficial in taking up of gene characterization and improvement of agronomic traits in guar.