The King's Official Birthday (alternatively the Queen's Official Birthday when the monarch is female) is the selected day in most Commonwealth realms on which the birthday of the monarch is officially celebrated in those countries. It does not necessarily correspond to the date of the monarch's actual birth.

The sovereign's birthday was first officially marked in the Kingdom of Great Britain in 1748, for King George II. Since then, the date of the king or queen's birthday has been determined throughout the British Empire and, later, the Commonwealth of Nations, either by royal proclamations issued by the sovereign or viceroy, or by statute laws passed by the local parliament.

The date of the celebration today varies as adopted by each country and is generally set around the end of May or start of June, to coincide with a higher probability of fine weather in the Northern Hemisphere for outdoor ceremonies. In most cases, it is an official public holiday, sometimes aligning with the celebration of other events. Most Commonwealth realms release a Birthday Honours list at this time.