The bacterium Streptococcus dysgalactiae is part of the group of streptococci, classified as Gram-positive cocci. Found mainly in the oral cavity, genitalia and in the skin of the mammary gland of animals and in its permanent environment, it is classified as an environmental pathogen, which is characterized as an opportunistic invader of the udder. Mastitis caused by this agent, usually presents in the clinical form of the disease, with infections of acute and severe character. The objective of this work was to identify the bacterial agent that causes bovine mastitis in the Northwest region of the State of Rio Grande do Sul, to isolate Streptococcus dysgalactiae and to determine its sensitivity profile against the main antimicrobials used in this region for the treatment of bovine mastitis. 917 mastitic milk samples were received for culture and antibiogram examination at the Veterinary Microbiology Laboratory of Unijuí, from January 2012 to May 2018. Streptococcus dysgalactiae, responsible for causing environmental mastitis, mainly of an acute nature, was isolated in 9 cases of mastitis, presenting a sensitivity profile, with high antimicrobial efficiency (100% sensitivity) for ciprofloxacin, cephalothin, enrofloxacin, gentamicin, norfloxacin and sulfazotrim. Thus, it is concluded that the prevalence of bovine mastitis caused by environmental agents, among them S. dysgalactiae in the Northwest region of the State of Rio Grande do Sul is high.