Republican Sinn Féin or RSF (Irish: Sinn Féin Poblachtach) is an Irish republican political party in Ireland. RSF claims to be heirs of the Sinn Féin party founded in 1905; the party took its present form in 1986 following a split in Sinn Féin. RSF members take seats when elected to local government in the Republic of Ireland, but do not recognise the validity of the Partition of Ireland. It subsequently does not recognise the legitimacy of the parliaments of Northern Ireland (Stormont) or the Republic of Ireland (Leinster House), so the party does not register itself with them.

The party emerged around the supporters of Ruairí Ó Brádaigh and Dáithí Ó Conaill. As Irish republican legitimists, they rejected the reformism of Gerry Adams and other members of Sinn Féin who supported abandoning the policy of abstentionism from the Oireachtas and accepting the legality of the Republic of Ireland. They support the Éire Nua policy which allows for devolution of power to provincial governments. RSF holds that the Irish Republic proclaimed in 1916 legally continues to exist, and that the Continuity Irish Republican Army Council is its de jure government.

The organisation views itself as representing "true" or "traditional" Irish republicanism, while in the mainstream media the organisation is portrayed as a political expression of "dissident republicanism". Republican Sinn Féin rejects the Good Friday Agreement and the Anglo-Irish Treaty; as part of this they assert that Irish republicans have the right to use militant means to "defend the Irish Republic" and considers the Continuity Irish Republican Army (IRA) to be the legitimate army of the Irish Republic, and the Continuity IRA Army Council its legal government.

The modern origins of the party date from the mid 1980s. The decision to form, reorganise or reconstitute, as its supporters see it, the organisation was taken in response to Gerry Adams-led Sinn Féin's decision at its 1986 ard fheis to end its policy of abstentionism and to allow elected Sinn Féin Teachtaí Dála (TDs) to take their seats in Leinster House's Dáil Éireann. Those who went on to form RSF opposed this move as it signalled a departure from the traditional republican analysis, which viewed the Dáil as an illegal assembly set up by an act of Westminster. They argued that republicans owed their allegiance to the Irish Republic, maintaining that this state existed de jure and that its authority rested with the IRA Army Council. (See: Irish republican legitimism) As Ruairí Ó Brádaigh declared:

With regard to councils, Sinn Féin has always been in the councils and that is as near to the enemy system that we dare to go. Sitting in Leinster House is not a revolutionary activity. Once you go in there, once you sign the roll of the House and accept the institutions of the state, once you accept their rulings, you will not be able to do it according to your rules. You will have to go according to their rules and they can stand up and gang up on you and put you out on the street and keep you out on the street.

And those in Leinster House, who have done everything; the firing-squads, the prison cells, the internment camps, the hunger strikes; the lot and weren't able to break this movement, that they can come and say "At last, we have them toeing the line, it took us 65 years, but they have come in from the cold, they have come in from the wilderness and we have them now." Never! That is what I say to you. Never!

Although it was passed by a two-thirds majority, those who went on to re-organise RSF claimed that the decision to end abstention was invalid under the Sinn Féin constitution, Section 1b of which stated: "No person ... who approves of or supports the candidature of persons who sign any form or give any kind of written or verbal undertaking of intention to take their seats in these institutions, shall be admitted to membership or allowed to retain membership." They pointed out that in their opinion the correct procedure was to drop or amend Section 1b of the constitution in one year, then come back the next year and propose entering Leinster House, when Section 1b was no longer in operation. In protest, they staged a walkout from the ardfheis and reconvened the ardfheis at another venue. RSF subsequently claimed that the delegates who had voted to drop abstentionism had in effect expelled themselves from the party. It is on this basis that RSF views itself as the only party entitled to the name of Sinn Féin and the sole legitimate successor to the original Sinn Féin established in 1905. Supporters of abstentionism also claimed that the vote at the ardfheis was gerrymandered. Journalist Ed Moloney points out that in 1986 the number of votes at the ardfheis, which reflects the size of Sinn Féin, almost doubled from 1985 to 1986, and then reverted to the 1985 level in 1987.

Adams-led Provisional Sinn Féin argued that a previous ardfheis in 1983 amended the constitution so that "no aspect of the constitution and rules be closed to discussion". This was done to enable the ardfheis to debate a motion to allow Sinn Féin candidates to stand in elections to the European Parliament and to take their seats if successful. Some argue that this argument is weakened, by the fact that candidature to the European Parliament had already been debated at the 1978 ardfheis, when a motion to stand candidates in the 1979 European elections was defeated at the Sinn Féin ardfheis. A vote to change abstentionism from a principle to a tactic failed to achieve a two-thirds majority vote in 1985. The results were 181 opposed and 161 in favour.