In Canada, the Leader of the Official Opposition in the Senate is the leader of the largest party in the Senate that is not in government. Since it is the Canadian House of Commons that determines which party is in government the size of party caucuses in the Senate bears no relation with which party forms the government side in the Senate and which party forms the opposition side. Thus the Leader of the Opposition in the Senate may actually lead more Senators than the Leader of the Government in the Senate. Therefore, the Leader of the Opposition in the Senate is the leader of the largest party in the Senate that is not in government. From 1993 until 2003 the Leader of the Opposition in the Senate was a Progressive Conservative despite the fact that the Progressive Conservatives were not the Official Opposition in the House of Commons, as the parties that were the Official Opposition (Bloc Québécois, Reform, Canadian Alliance) did not have Senate representation.

The counterpart in the lower house is the Leader of the Opposition in the House of Commons. Since that person is usually also the party leader, he or she appoints his or her counterpart in the Senate except when the Official Opposition in the House is not the same as the Official Opposition in the Senate as was the case from 1993 to 2003 when the leader of the Progressive Conservative Party in the Canadian House of Commons chose the Leader of the Opposition in the Senate.

See also List of Canadian Leaders of the Opposition in the Senate, Leader of the Government in the Senate (Canada)