The positive result on Friday came on a third round of testing of the animal, which had previously tested negative to a different type of confirmatory test, a statement from the US Department of Agriculture (USDA) said.
"Because of the conflicting results ... a sample from the animal will be sent to the OIE-recognised reference laboratory for BSE in Weybridge, England. USDA will also be conducting further testing, which will take several days to complete," it said.
The animal was non-ambulatory - a so-called downer animal - and thus banned from human consumption. There is no chance its meat had entered the human food supply, officials said.
The announcement came the same day that Portugal announced its first suspected case of the human form of mad cow disease, while France said it had identified its 13th case of the degenerative brain ailment.
Variant Creutzfeldt-Jakob disease (vCJD) is a human form of bovine spongiform encephalopathy (BSE) - commonly called mad cow disease - caused by a rogue protein that proliferates in the brain, turning it spongy.
Britain was the epicentre of the BSE outbreak that occurred in the late 1990s. Its suspected source was cattle feed that came from cows with brain disease.
The pathogen leapt the species barrier to humans through the consumption of contaminated beef, experts believe.
US beef ban
Japan, previously the top export market for US beef, halted imports of US beef in December 2003 after a cow infected with BSE was discovered in the US state of Washington.
The US has applied intense pressure on Japan to resume imports of US beef, but Japan has so far resisted the call amid further safety assessments.
"We received final results a short time ago. Of the three samples, two were negative, but a third came back reactive"
The USDA said on Friday it had tested more than 375,000 animals for BSE since June 2004.
Three animals tested inconclusive and were subjected to immunohistochemistry or IHC testing, with negative results, the USDA said.
But earlier this week, the department's inspector-general recommended further checking of the meat samples using a second internationally recognised test, the SAF immunoblot or Western blot test.
"We received final results a short time ago. Of the three samples, two were negative, but a third came back reactive," the USDA statement said.
The animal was processed at a facility that handles only animals unsuitable for human consumption, and its carcass was incinerated, the department added.