An appropriation bill, also known as supply bill or spending bill, is a proposed law that authorizes the expenditure of government funds. It is a bill that sets money aside for specific spending. In some democracies, approval of the legislature is necessary for the government to spend money.

In a Westminster parliamentary system, the defeat of an appropriation bill in a parliamentary vote generally necessitates either the resignation of a government or the calling of a general election. One of the more famous examples of the defeat of a supply bill was the 1975 Australian constitutional crisis, when the Senate, which was controlled by the opposition, refused to approve a package of appropriation and loan bills, prompting Governor-General Sir John Kerr to dismiss Prime Minister Gough Whitlam and appoint Malcolm Fraser as caretaker Prime Minister until the next election (where the Fraser government was elected).

An appropriation bill is a bill that authorizes the government to withdraw funds from the Consolidated Fund of India for use during the financial year. Although Appropriation Acts are not included in any official list of central laws, they technically remain on the books. Since 2016, appropriation bills in India include a sunset clause as result of which the Act is automatically repealed after its purpose is met. Appropriation Acts passed prior to 2016 were repealed by the enactment of The Appropriation Acts (Repeal) Act, 2015 in April 2016.

In New Zealand, an Appropriation Bill is the formal name for the annual Act of Parliament that gives legal effect to the New Zealand Budget, that is, the Government's taxing and spending policies for the forthcoming year (from 1 July to 30 June). Like other bills, it is enacted, following debate, by the House of Representatives, and assented to by the governor-general. The main Appropriation Bill is traditionally placed before the House for its first reading in May amid considerable media interest, an event known as the introduction of the Budget. An Appropriation Bill is not sent to a select committee, a lengthy process undergone by most bills during which they are scrutinised in detail by the committee, which also receives public submissions relating to the bill. Instead, an expedited process is followed in which the Appropriation Bill essentially goes directly to its second reading for consideration by the committee of the whole House. Royal assent is granted after the formality of a third reading.

The main Appropriation Bill is formally called an "Appropriation (Estimates) Bill", or, after assented to, an "Appropriation (Estimates) Act". Supplementary Budgetary legislation in New Zealand includes an annual "Appropriation (Confirmation and Validation) Bill", which serves to validate taxation and spending incurred in the previous year which fell outside the previous year's Budget, and "Imprest Supply Bills," typically several in a year, which grant interim authority to the Government to tax and spend.

Both Appropriation and Imprest Supply bills fall under the rubric of confidence and supply. A refusal by the House to pass such a Bill conventionally leads to either the resignation of the Government (unlikely, since there is usually no alternative Government immediately available) or to a dissolution of the House and a subsequent general election.

In the Philippines, the Congress which consists of the House of Representatives and the Senate is mandated to pass a General Appropriation Bill annually. If the Congress fails to pass a General Appropriation Bill for a fiscal year, the General Appropriation Act for the previous fiscal year would be used until a bill for the specific year is passed.

Under the presidential system, the support of the Congress for the President's appropriations requests is not necessary for the separately-elected President to remain in office, but it can severely limit the President's ability to govern effectively.