The Leader of the Opposition in South Australia is the leader of the largest minority political party or coalition of parties, known as the Opposition, in the House of Assembly of the Parliament of South Australia. By convention, the leader of the opposition is a member of the House of Assembly. The leader acts as the public face of the opposition, and acts as a chief critic of the government and ultimately attempt to portray the opposition as a feasible alternate government. They are also given certain additional rights under parliamentary standing orders, such as extended time limits for speeches. Should the opposition win an election, the Leader of the Opposition will be nominated to become the Premier of South Australia.

Sir John Blackler Colton, was an Australian politician, Premier of South Australia and philanthropist. His middle name, Blackler, was used only rarely, as on the birth certificate of his first son.