Brucellosis is a worldwide zoonotic disease of socio‐economic importance. Understanding the association of this disease with pregnancy outcome has the potential of contributing to the reduction of its reproductive burden in humans and animals among pastoral communities in Tanzania. A prospective cohort study was conducted in Kagera Region on pregnant women (n = 76) and gravid ruminants (121 cattle, 125 goats and 111 sheep). Exposed and non‐exposed groups to brucellosis were followed for 6 months (from 15 November 2017 to 15 April 2018). Sera were collected and analysed using Rose Bengal Test (RBT) and Fluorescence polarization assay (FPA) test. Measures of effect, univariable and multivariable logistic regression analyses were computed. Positivity to both RBT and FPA tests was 21% (95% CI: 12.5–32) in pregnant women and 5% (95% CI: 3.1–8) in gravid ruminants. Among aborted cases, four women (out of nine), two cows (out of seven), two goats (out of 26) and zero sheep (out of 11) were positive to brucellosis. The abortion rate in humans and ruminants was 11.8% and 12.3%, respectively. Seropositivity to brucellosis was similar in aborted and non‐aborted cases in humans (p = .08) and in ruminants (p = .2). At the population level, brucellosis was associated with abortions (population attributable risk: PAR) at 3.5% in pregnant women and at 0.5% in gravid ruminants in the study area. Infections to brucellosis were increased in exposed pregnant women (OR = 19; 95% CI: 1.8–203, p = .01) and in cattle (OR = 11; 95% CI: 1.3–88, p = .02). There is an indication that brucellosis could be contributing to abortions in pregnant women and domestic ruminants Kagera Region. Molecular tools could support more the results from serological tests to avoid cross‐reaction with other pathogen agents. Control of brucellosis in animals is likely to reduce the threat of abortions in humans.