The Kilmainham Treaty was an agreement reached in May 1882 between the United Kingdom Government under William Ewart Gladstone and the Irish nationalist leader Charles Stewart Parnell. It was seen as a major triumph for Irish nationalism as it managed to win abatement for tenant rent-arrears from the Government at the height of the Land War.

The agreement extended the terms of the Second Land Act of 1881, with which Gladstone intended to make broad concessions to Irish tenant farmers. But the Act had many weaknesses and failed to satisfy Parnell and the Irish Land League because it did not provide a regulation for rent-arrears or rent-adjustments (in the case of poor harvests or deteriorated economic conditions).

After it became law 22 August 1881 Parnell in a series of speeches in September and October launches violent attacks on William Forster the Chief Secretary for Ireland and even on Gladstone. Gladstone warned him not to frustrate the Act, but Parnell repeated his contempt for the Prime Minister. On 12 October the Cabinet, fully convinced that Parnell was bent on ruining the Act, took action to have him arrested the following day in Dublin.

Parnelll was conveyed to Kilmainham Gaol where he would join several other prominent members of the Land League who had also protested against the Act. He was well aware that not all of the Liberal Cabinet - in particular Joseph Chamberlain were in favour of mass internment of suspects as then taking place across Ireland under the Irish Coercion Act. The repressions did not have the desired effect, the predicted improvements expressed by W. E. Forster did not materialize. It resulted in him becoming isolated within the Cabinet. Coercion was increasingly unpopular with the Liberal Party, now that it did not appear to be working.

In gaol Parnell had begun to turn over in his mind the possibility of coming to an arrangement with the Government. He had been corresponding with Mrs Katharine O'Shea who engaged her husband Captain O’Shea in April 1882 to act as a go-between for negotiations on behalf of Parnell. O’Shea contacted Gladstone on 5 May having been informed by Parnell that if the Government would settle the rent-arrears problem on the terms he proposed, he was confident that he would be able to curtail outrages. He further urged for the quick release of the League’s organizers in the West, Sheridan and Boyton, who would then work for pacification. This shocked Forster, but impressed Gladstone.

Accordingly on 2 May Gladstone informed the House of Commons of the release of Parnell and the resignation of Forster (who was replaced by Lord Frederick Cavendish). Gladstone always denied there had been a ‘Kilmainham Treaty’, merely accepting that he ‘had received information’s. He kept his side of the arrangement by subsequently having the Arrears of Rent (Ireland) Act 1882 enacted.

The fact that it was called a "treaty" is important. Since treaties were usually signed between two nations, it began to spread the possibility and the idea that Ireland could become a country independent from Britain. Parnell managed to place a spin on the treaty itself so that it became popular in Ireland and strengthened Irish nationalism, since he apparently managed to force concessions from the British while in gaol. After the’ treaty’ was agreed, those imprisoned with Parnell were then released from gaol. This transformed Parnell from a respected leader to a national hero.