The exact virus strain is not known, but further testing is underway at the European Union's bird flu laboratory in Weybridge, Surrey, in southeast England.
Preliminary tests had found "highly pathogenic H5 avian flu" in a sample from the bird, which was found in the Fife region, north of the capital Edinburgh, health officials said in a statement.
The dead swan was found in the coastal area of Cellardyke, and the Scottish Executive said restrictions have been put in place around the area.
"In accordance with a recent EU decision the Scottish Executive is putting in place a protection zone of a minimum of three kilometres radius and a surveillance zone of 10 kilometres," said an executive spokesman, according to the BBC's website.
"Keepers of birds in the protection zone are being instructed to isolate their birds from wild birds, by taking them indoors where ever possible."
Measures to restrict the movement of poultry, eggs and poultry products from these zones will also be brought into effect immediately.
Further restrictions such as housing and movement controls may be put in place if tests confirm it is the H5N1 strain, he said.
If the lethal H5N1 strain is confirmed, Britain will be the 13th country in the EU the virus has been found in wild birds.
However officials have stressed that there is no reason for public health concern.
BBC News reported that the bird was found on its own near the shoreline and was in an emaciated state.
A leading British virologist, John Oxford, said the dead swan is an indication that many more wild birds in the vicinity are likely to be infected.
The announcement came as Britain's Department for Environment, Food and Rural Affairs began a two-day exercise to test the country's response to a possible outbreak of bird flu.
Bird farm infection
Meanwhile, the H5N1 strain has been detected for the first time at a poultry farm in Germany.
The national veterinary laboratory confirmed that the farm in Wermsdorf near Leipzig has suffered an outbreak of the virus, following an outbreak in neighbouring France in February.
Local authorities said a special protection zone of three kilometres has been drawn around the poultry farm, and farm staff have been ordered to slaughter all its 16,400 fowl within two days, according to Ingolf Herold, a member of the veterinary authorities in Saxony.
H5N1 bird flu has dealt a blow to the French poultry industry. The only other EU country where the disease is suspected to have spread to farm poultry is Sweden.
The affected farm in Wermsdorf had mainly turkeys but also geese and chicken. Most of the 1,400 turkeys on the part of the farm where the sick ones were found have already been killed, said Mr Herold.
Another human death
Cambodia has registered its second human bird flu victim this year, with a 12-year-old boy succumbing to the virus.
Authorities said testing done by a Phnom Penh laboratory found he had died from H5N1, after reportedly eating sick chickens. The boy was from Prey Veng province, about 90 kilometres east of Phnom Penh.
Meanwhile in India, testing has found that bird flu has spread to more towns and villages in the western state of Maharashtra.