ABSTRACT The rhamnogalacturonan I (RGI) is part of pectin plant cell wall, and currently there is scarce information about the elicitor effect of its fragments in the fruit defense mechanism. In this work, the effect of the fragments of RGI on the defense mechanism in tomato (Solanum lycopersicum L.) fruits was evaluated. Three fragments of RGI released from enzymatic degradation by RG lyase were characterized according to their physical-chemical and structural characteristics. Two of RGI fragments (F-1 and F-2) were selected and applied on tomato fruits during 48 h. At different exposure times (0, 0.5, 1, 6, 24, and 48 h) tomato samples were taken, and the enzymatic activity of chitinase was assessed by fluorometry, and β-1,3-glucanase and peroxidase by spectrophotometry (UV-visible). The absorption bands determined by Fourier transform infra-red (FT-IR) spectroscopy in the RGI fragments, was different concerning RGI no fragmented. The RGI had a molecular weight of 64 kDa and an intrinsic viscosity of 40.9 mL g-1; while the molecular weight of F-1 and F-2 fragments were 20 and 16 kDa, and an average intrinsic viscosity of 27 and 26 mL g-1, respectively. The content of galacturonic acid decreased, and the neutral sugars increased concerning the time of enzymatic degradation. F-1 and F-2 fragments induced an increase in the level of β -1,3-glucanase activity in tomato fruits; however, an increase in the enzymatic activity of chitinase and peroxidase was observed after 0.5 h of exposure to F-2 fragment. It was concluded that the RGI fragments were structurally different from the RGI no fragmented and induced the natural defense mechanism of the tomato fruit.