The present study evaluated the toxicogenetic potential of the water and sediment from Paraopeba River, five days after the rupture of the mining tailings dam at Brumadinho (Minas Gerais, Brazil), on January 2019, by Allium cepa test. Water samples collected at sites closest to the collapsed dam caused a higher frequency of genetic damage and lower cell division rate compared to the upstream site. In sediment it was observed a decrease in the cell division rate and genetic damage in the sampling sites closest to the dam. The frequency of aberrations and the mitotic index in A. cepa cells exposed to sediments were negatively associated with the levels of most elements in total and bioavailable fractions. A negative association between some metal concentrations in total and dissolved water with the mitotic index was also observed. Otherwise, the genotoxic effects were positively correlated with the concentrations of most metals. The results demonstrate the potential of the tailings to cause cytogenotoxic effects due the higher metal concentrations released in water column and in sediments. This is the first report highlighting the toxicogenetic potential of the released tailings and reinforce the need for a long-term monitoring of the watershed due the advance of the tailings plume through Paraopeba River over time. Display omitted •After dam rupture, 12 million m3 of mining tailings reached the Paraopeba River.•Water and sediment from river promoted genotoxic effects in Allium cepa.•Cytogenotoxicity was inversely associated with the metals in sediment.•Genotoxic effects were positively associated with toxic metals in water.•Genotoxicity in sites yet not reached by tailing indicates previous contamination.