The Statute of Westminster 1931 is an act of the Parliament of the United Kingdom that sets the basis for the relationship between the Commonwealth realms and the Crown.Passed on 11 December 1931, the statute increased the sovereignty of the self-governing Dominions of the British Empire from the United Kingdom. It also bound them all to seek each other's approval for changes to monarchical titles and the common line of succession. The statute was effective either immediately or upon ratification. It thus became a statutory embodiment of the principles of equality and common allegiance to the Crown set out in the Balfour Declaration of 1926. As the statute removed nearly all of the British parliament's authority to legislate for the Dominions, it had the effect of making the Dominions largely sovereign nations in their own right. It was a crucial step in the development of the Dominions as separate states.

Its modified versions are now domestic law within Australia and Canada; it has been repealed in New Zealand and implicitly in former Dominions that are no longer Commonwealth realms.