The Referendum Act 1975 (c. 33) also known at the time as the Referendum Bill was an act of the Parliament of the United Kingdom, which made legal provision for the holding of a consultative referendum on whether the United Kingdom should remain a member of the European Communities (EC)—generally known at the time in the UK, with reference to their main component, the European Economic Community (EEC) as stipulated in the Act, also known at the time as the "Common Market". The bill was introduced to the House of Commons by the Leader of the House of Commons and Lord President of the Council Edward Short on 26 March 1975; on its second reading on 10 April 1975, MPs voted 312–248 in favour of holding the referendum—which came the day after they voted to stay in the European Communities on the new terms set out in the renegotiation.

The Act gave effect to a manifesto commitment of the Labour Party at the general election of October 1974, and set out the arrangements and procedure for the United Kingdom's first ever national referendum, in which voting was to take place simultaneously in all parts of the country. The Act was given Royal Assent on 8 May 1975 and came into effect immediately on the same date.

In accordance with the Act, the European Communities membership referendum took place on Thursday 5 June 1975, and voters approved continued EC/EEC membership by 67% to 33% on a national turnout of 64%.

The Act became effectively spent following the declaration of the national referendum result; however, it was not officially repealed until eleven years later, by the Statute Law (Repeals) Act 1986.

The Act legislated for a non-binding referendum to be held in the United Kingdom on Thursday 5 June 1975 on the issue of continuing membership of the EC and the EEC (the Common Market), which was to be a single majority vote, to be overseen by an appointed "Chief counting officer" who would declare the final result for the United Kingdom. As there was no previous precedent for the holding of any such plebiscite across the United Kingdom the Act also set out its procedure and format.

The Act also provided for the equal public funding of two campaigns, one for a Yes vote, the other for a No vote. While each campaign was to receive the same amount of public money which amounted to £125,000 each, other donations were also to be permitted with no upper spending limit for either side .

The Act allowed for the appointment of a "National Counting Officer" (later "Chief Counting Officer") who would lead and oversee the referendum both centrally and nationally. Also within the legislation it oversaw the appointment of the "Returning Officers" who would oversee the counts within their local areas. The Act legislated for verification which would be held at local authority level (district councils) after the close of polls, but the counting of votes would be only permitted and declared at the level of administrative regions under the Local Government Act 1972 and the Local Government (Scotland) Act 1973 as follows:

This meant there would be a total of 68 counting areas across the United Kingdom (47 in England, eight in Wales, twelve in Scotland, and a single area for Northern Ireland). Verification took place locally in England and Wales after the polling stations closed, but the referendum count itself did not begin until the day following the poll, Friday 6 June, beginning at 09:00 BST.