United Nations Security Council resolution 1540 was adopted unanimously on 28 April 2004 regarding the non-proliferation of weapons of mass destruction. The resolution establishes the obligations under Chapter VII of the United Nations Charter for all member states to develop and enforce appropriate legal and regulatory measures against the proliferation of chemical, biological, radiological, and nuclear weapons and their means of delivery, in particular, to prevent the spread of weapons of mass destruction to non-state actors.

It is notable in that it recognizes non-state proliferation as a threat to the peace under the terms of Chapter VII of the United Nations Charter, and creates an obligation for states to modify their internal legislation.

Furthermore, the resolution requires every state to criminalize various forms of non-state actor involvement in weapons of mass destruction and its related activities in its domestic legislation and, once in place, to enforce such legislation. By virtue of its universal scope and mandatory nature, resolution 1540 marks a departure from previous nonproliferation arrangements and adds a novel layer to the nonproliferation regime. Before the resolution was adopted, the non-proliferation regime was based on many partly overlapping arrangements, none of which established universal mandatory obligations.

Resolution 1540 was adopted under Chapter VII of the United Nations Charter, which covers "threats to the peace, breaches of the peace and acts of aggression." The resolution itself states in its opening paragraph: "proliferation of nuclear, chemical and biological weapons, as well as their means of delivery, constitutes a threat to international peace and security". Even though the security council had already recognized the proliferation of WMD as a threat to international peace and security in the UNSC Presidential Statement on 31 January 1992, it took the council until 2004 to adopt such a decisive resolution.

1540 is only the second resolution to invoke Chapter VII without relating the fact to a specific time and place, the first being United Nations Security Council Resolution 1373, which was voted in the aftermath of the September 11 attacks as an attempt to counter international terrorism. The possibility of terrorists accessing WMD was already considered in Resolution 1373 paragraph 3a and 4, and UNSCR 1540 emerged from those two paragraphs.

The three main obligations created by the resolution are:

The resolution also emphasises the continuing importance of non-proliferation and disarmament agreements, and provides for the creation of a committee (the 1540 Committee) to oversee the implementation of the resolution. States are called upon to provide reports to the 1540 Committee on their current situation and future plans vis-à-vis the obligations laid out in the resolution within six months of the adoption of the resolution, though in reality many states took much longer.

The 1540 Committee was deliberately created without the power to impose sanctions. While the resolution is, in theory, obligatory for all members of the UN, it was decided that the implementation process should be based on collaboration and participation rather than enforcement. To further these ends, one of the main functions of the committee has been as a "clearing house" of offers and requests for assistance in fulfilling obligations created by the resolution.