ABSTRACT.- Leptospira kirschneri is an emerging human and animal pathogen. Here we describe 2 unrelated outbreaks of acute fatal leptospirosis caused by L. kirschneri in suckling lambs and calves diagnosed following a flooding event in the spring of 2023 in northern Uruguay. In outbreak 1, the sheep flock was raised in a low-lying area previously used for rice cultivation; 20 lambs died after developing apathy, jaundice, and hemoglobinuria. In outbreak 2, 2 calves were found dead. At autopsy, 4 lambs and 2 calves had marked jaundice, hemoglobinuria, dark-red kidneys, and hepatomegaly. Microscopically, we found marked hemoglobinuric nephrosis, lymphohistiocytic tubulointerstitial nephritis, hepatocellular dissociation, and random hepatocellular necrosis with periportal lymphohistiocytic hepatitis and canalicular cholestasis. Positive immunostaining for Leptospira sp. was found in the liver (lamb C), both the liver and kidney (calf B), and the liver (calf A). Leptospira kirschneri was PCR-confirmed using kidney and liver samples from the autopsied animals. Acute leptospirosis caused by L. kirschneri has not been reported previously in sheep and cattle, to our knowledge. Our findings highlight that L. kirschneri infection can cause acute leptospirosis in unvaccinated lambs and calves. © 2025 The Author(s).