DUP MP Gregory Campbell has written to the US Ambassador to object to the removal of Scots-Irish as a distinct ancestry by the USA Census Bureau.
Individuals in the USA who report themselves as Scots-Irish in the American Community Survey will now be included in the 'other groups' category.
The census data will retain distinct categories for Irish, English, Scottish and Welsh.
The Scots-Irish, referred to in the British Isles as Ulster-Scots, are the descendants of those who came to Ireland during the Plantation of Ulster in the 17th century.
Mr Campbell said: "I have written to the US Ambassador in London, Mr Louis Susman, requesting that the US government reverse this decision and continue to have Scots-Irish available in US census data.
"I have also informed Scotland's Westminster Members of Parliament about this matter."
"I indicated to the US Ambassador that it is an historical fact that immigrants of Scottish descent from the Ulster region of Ireland settled and helped to establish and build the United States of America.
"Scots-Irish descendents have made a significant contribution to society since the foundation of the USA in areas such as music, film, military and political life of the nation. There have been numerous famous Scots-Irish among 17 US Presidents."
The East Londonderry MP also indicated that the tourism industry in Northern Ireland might suffer as a result of the decision.
"Much of the tourism between the USA and Northern Ireland has been aimed at the Scots-Irish because of the ancestry links.
"People from a Scots-Irish descent in the USA are a separate ancestry as distinct from an Irish descent."
It is estimated that at least 200,000 Scots-Irish emigrated to North America, primarily during the colonial era.
Many figures in US history have come from the Scots-Irish, including three Presidents whose parents were born in Ulster. Andrew Jackson's parents emigrated from Boneybefore in Co Antrim; James Buchanan's parents were from near Omagh, and the father of Chester Alan Arthur emigrated from Cullybackey, Co Antrim.