Introduction Toxoplasma gondii is a widely distributed parasite and of great importance to human and animal health. Methods The objective of this study was to assess the prevalence of T. gondii antibodies and risk factors associated with the infection in sheep in the Northwest region of the State of Rio Grande do Sul, Brazil; this region has a very high rate of human ocular toxoplasmosis. Ovine sera were tested by the modified agglutination test (cut-off 1:25). Results T. gondii antibodies were detected in 70.2% (224 of 319). According to the logistic regression, the most significant factors associated were age and cat access to food stock facility. Conclusion Preventive measures are discussed to reduce the risk of transmission of this zoonosis.