Lech Aleksander Kaczyński (Polish pronunciation:   ( listen); born 18 June 1949) is the President of the Republic of Poland, a politician of the party Prawo i Sprawiedliwość (Law and Justice, PiS). Kaczyński served as Mayor of Warsaw from 2002 until 22 December 2005, the day before his presidential inauguration. He is the identical twin brother of the former Prime Minister of Poland and current Chairman of the Law and Justice party, Jarosław Kaczyński.

Lech Kaczyński was born in Żoliborz, Warsaw, the son of Rajmund (an engineer who served as a soldier of the Armia Krajowa in World War II and a veteran of the Warsaw Uprising) and Jadwiga (a philologist at the Polish Academy of Sciences). As a child, he starred in a 1962 Polish film, The Two Who Stole the Moon (Polish title O dwóch takich, co ukradli księżyc) with his twin brother Jarosław.

Lech Kaczyński is a graduate of law and administration of Warsaw University. In 1980 he was awarded his PhD by Gdańsk University. In 1990 he had his habilitation in labour and employment law. He later assumed professorial positions at Gdańsk University and Cardinal Stefan Wyszyński University in Warsaw.