John Joseph Clancy (15 July 1847 - 25 November 1928), usually known as J. J. Clancy, was an Irish nationalist politician and MP. in the House of Commons of the United Kingdom of Great Britain and Ireland and member of the Irish Parliamentary Party for North County Dublin from 1885 to 1918 in the Parliament of the United Kingdom, one of the leaders of the later Irish Home Rule movement and promoter of the Housing of the Working Classes (Ireland) Act 1908, known as the Clancy Act. He was called to the Irish Bar in 1887 and became a KC (King’s Counsel) in 1906.

John Joseph Clancy (c.1891/92 - 1 May 1932) was an Irish politician and Sinn Féin Teachta Dála of the First Dáil for Sligo North from 1918 to 1921. In the general election of 1918, he was elected as part of the Sinn Féin landslide, defeating the Nationalist Thomas Scanlan who had sat for the Sligo North seat since 1909, by 9,030 to 4,242. Like the other Sinn Féin members, Clancy did not take his seat at Westminster but took part in the revolutionary First Dáil in Dublin. He sat for only one term and did not stand for election again.

Clancy was also Secretary to the Sligo County Committee of Agriculture, and Chairman of Sligo County Council. At the first meeting of the County Council on 22 June 1920 he said that their work as a council would be guided towards clearing Britain out of Ireland. He later served as an officer in the Irish Free State Army.

He was imprisoned several times and was a hunger striker. He was arrested and interned in Usk prison in May 1918. He was arrested twice in 1919, on 15 January and 7 April, and was already serving a term of three months imprisonment when he was sentenced to a further three months with hard labour on 6 June 1919, for unlawful assembly.

On 3 May 1932, when he was 40, his body was found in the River Shannon at Limerick. He had gone to Mass in Tipperary on Sunday 1 May but had not returned. The verdict at the inquest on 4 May was that he had drowned accidentally. He was married, and his funeral in Tipperary on 5 May was attended by his widow and four children. He was interred at the family burial place at Ardmoyle, Co. Tipperary.

He should not be confused with the Irish Nationalist politician J. J. Clancy (1847–1928), who was MP for North County Dublin, 1885–1918.