A casting vote is a vote that someone may exercise to resolve a tied vote in a deliberative body. A casting vote is typically by the presiding officer of a council, legislative body, committee, etc., and may only be exercised to break a deadlock.

Examples of presiding officers who hold casting votes are the Speaker of the House of Commons in the United Kingdom and the President of the United States Senate (an ex-officio role of the Vice President of the United States).

In some legislatures, a casting vote may be exercised however the presiding officer wishes. For example, the Vice President of the United States may exercise their casting vote when the Senate is evenly divided according to their own personal beliefs; by virtue of the Vice President's political leanings and affiliations, the Vice President's political party is able to serve as the majority party in the Senate and elect one of their own to serve as Majority Leader.

In some other legislatures, by contrast, a casting vote can only be exercised according to strict rules or constitutional conventions. For example, the Speaker of the British House of Commons (a position whose functions and conventions of operation inspire similar roles in several other nations using the Westminster system) is expected by convention to follow Speaker Denison's rule (i.e. to vote to allow further discussion, if this is possible, and otherwise to vote in favour of the status quo). This in effect means "Yes/Yea/Aye" on the first and second reading of a bill, "No/Nay" on the third, "Yes/Yea/Aye" on the government's budget, and "No/Nay" on a motion of no confidence.

In the United States, the concept of a casting vote is not used in Robert's Rules of Order.