Jarosław Aleksander Kaczyński (Polish pronunciation: [jaˈrɔswaf kaˈt͡ʂɨj̃skʲi] (listen); born 18 June 1949) is a Polish politician who is currently serving as leader of the Law and Justice party (known by its Polish acronym PiS), which he co-founded in 2001 with his twin brother, Lech Kaczyński, who served as president of Poland until his death in 2010.

Running for PiS, he served as the prime minister of Poland from July 2006 to November 2007, while his brother was the president of Poland. After PiS's electoral defeat in 2007, Kaczyński was the main leader of the opposition during Civic Platform's governments. Following the death of his brother in a plane crash, Jarosław Kaczyński ran in the 2010 Polish presidential election losing to Bronisław Komorowski.

Since the 2015 victories of PiS, both in the presidential and parliamentary election, Kaczyński is considered to be the most powerful politician in Poland and one of the most influential European leaders. For this reason, in Poland he is called by some people the "Chief of State" (following the example of Józef Piłsudski). In 2020, he was designated as the Deputy Prime Minister of Poland with oversight over the defense, justice and interior ministries.