The Leader of the Opposition in the House of Representatives, more commonly described as the Leader of the Opposition, is the politician in the politics of Thailand who leads the main minority party in the House of Representatives. The Leader of the Opposition is the leader of the largest political party in the House of Representatives that is not in government.

The House of Representatives is the lower house of the National Assembly of Thailand, the legislative branch of the Thai government. The system of government of Thailand is that of a constitutional monarchy and a parliamentary democracy. The system of the Thai legislative branch is modelled after the Westminster system. The House of Representatives has 500 members, of which 400 are elected through single member constituency elections, while the other 100 are chosen through party lists parallel voting.