The fragment was discovered in a sand and gravel pit in Russia's south Siberian republic of Gorny Altai, RIA Novosti reported Wednesday.
A huge flood may have killed the prehistoric animal about 18,000 years ago when a large lake in the Chui Steppe overflowed its banks, experts said.
"That caused the most massive flood in history, with water moving at a speed of 160 kilometers (99 miles) an hour and covering earth with 490 meters (1,607 feet) of water," the republic's government said in a report on its website.
Remains of extinct animals have previously been found in Altai; in 2012 tusks of mammoths, which died out about 10,000 years ago during the last ice age, were discovered buried 30 feet below the ground.
The recent horn find will be examined then handed over to one of the republic's museums, the Altai government said.