Guar gum (GG) was incorporated into soy protein isolate (SPI) films using a blending solution casting method to form SPI/GG composite films. The effects of SPI and GG contents on the transparency, water susceptibility, mechanical, and gas‐barrier properties of SPI/GG composite films were analyzed. The results showed that SPI/GG composite films with added GG were much more tensile‐resistant, water‐resistant, gas‐barrier properties but less deformable property than SPI control film. The presence of GG also improved film barrier to the light. The analysis results of contact angle measurement, Fourier transform infrared spectroscopy, and scanning electron microscope indicated that GG induced increased network compactness of the composite films which resulted from strong intermolecular interactions, such as hydrogen bonding, that existed between SPI and GG. Findings indicate that GG may be used as a natural means to improve specific properties of SPI films. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43382.