Soybeans (Glycine max (L.) Merril) (Fabaceae) are the most important grain crop in Rio Grande do Sul State, Brazil. Recently,significant infestations of phytophagous mites were observed in different regions of this state. This research aimed to identify the mite fauna associated with the soybean crop grown in the northwestern region of Rio Grande do Sul, particularly in Três de Maio and Mato Queimado counties. Assessments were made fortnightly in transgenic soybeans with irrigation (L1A) and without irrigation (L2A and B), in conventional soybeans without irrigation (L3C and D), and in edge plants (L1, 2 and 3). In soybean plants, we found 18,100 mites belonging to five families, nine genera and 12 species, besides those of Oribatida. Area L1A showed higher richness and abundance, with 10 species, followed by area L2B with insecticide application, with nine species. Lower richness was observed in L2A, with five species, and L3D showed lower abundance. Phytoseiidae showed higher richness, with five species, followed by Tetranychidae, with four species. Tetranychus urticae Koch, Mononychellus planki McGregor and Tetranychus ludeni Zacher were the most common phytophagous mites on soybean plants. Neoseiulus idaeus Denmark & Muma, Pseudopronematus sp., N. californicus McGregor and N. anonymus Chant & Baker. N. idaeus were the most common predators. On edge plants, T. ludeni and T. urticae were the most abundant, while N. idaeus and Agistemus riograndensisJohann & Ferla stood out among the predatory mites.