The emperor of Japan is the hereditary monarch and head of state of Japan. The emperor is defined by the Constitution of Japan as the symbol of the Japanese state and the unity of the Japanese people, his position deriving from "the will of the people with whom resides sovereign power". The Imperial Household Law governs the line of imperial succession. Pursuant to his constitutional role as a national symbol, and in accordance with rulings by the Supreme Court of Japan, the emperor is personally immune from prosecution. By virtue of his position as the head of the Imperial House, the emperor is also recognized as the head of the Shinto religion, which holds him to be the direct descendant of the sun goddess Amaterasu. According to tradition, the office of emperor was created in the 7th century BC, but the first historically verifiable emperors appear around the 5th or 6th centuries AD.

The role of the emperor of Japan has historically alternated between a largely ceremonial symbolic role and that of an actual imperial ruler. Since the establishment of the first shogunate in 1192, the emperors of Japan have rarely taken on a role as supreme battlefield commander, unlike many Western monarchs. Japanese emperors have nearly always been controlled by external political forces, to varying degrees. Between 1192 and 1867, the shōguns, or their shikken regents in Kamakura (1203–1333), were the de facto rulers of Japan, although they were nominally appointed by the emperor. After the Meiji Restoration in 1868, the emperor was the embodiment of all sovereign power in the realm, as enshrined in the Meiji Constitution of 1889. Since the enactment of the 1947 constitution, the role of emperor has been relegated to that of a ceremonial head of state without even nominal political powers. The emperor is the head of the Japanese honors system, conferring orders, decorations, medals, and awards in the name of the state and on behalf of its people in accordance with the advice of the Cabinet.

Since the mid-nineteenth century, the emperor and other members of the imperial family have resided at the Imperial Palace, located on the former site of Edo Castle in the heart of Tokyo, the current capital of Japan. Earlier, emperors resided in Kyoto, the ancient capital, for nearly eleven centuries. The Emperor's Birthday (currently 23 February) is a national holiday.

Naruhito is the current emperor of Japan. He acceded to the Chrysanthemum Throne upon the abdication of his father, Emperor Akihito, on 1 May 2019. He is the only remaining monarch and head of state in the world who holds the title of Emperor.

Most constitutional monarchies formally vest executive power in the reigning monarch in their capacity as head of state. In turn, these constitutional monarchs are bound by either convention or statute to exercise their prerogatives on the advice of ministers responsible to the duly elected parliament. Some monarchies codify this principle further by requiring royal acts to be countersigned by a minister in order to take effect, thus passing political and legal accountability from the reigning monarch to the minister tendering advice. By contrast, Japan is one of only two such sovereign states where the monarch is not even the nominal chief executive; the other is Sweden. Rather, Article 65 of the Constitution of Japan explicitly vests executive authority in the Cabinet, of which the prime minister is the leader. The emperor is also not the commander-in-chief of the Japan Self-Defense Forces. Instead, the Japan Self-Defense Forces Act of 1954 explicitly vests supreme command and control in the prime minister. Nevertheless, the emperor remains Japan's internationally recognized head of state.

The emperor's fundamental role within the machinery of the Japanese constitution is to perform important representational functions as "…the symbol of the State and of the unity of the People, deriving his position from the will of the people with whom resides sovereign power." He is limited to performing "acts in matters of state" as delineated by the Constitution, without even nominal powers related to government. Moreover, said acts are only exercised in accordance with the binding advice and consent of the Cabinet, which is collectively responsible to the Diet and thence to the electorate. Nevertheless, the emperor enjoys the following rights in the conduct of state business: (a) the right to be consulted before acting on ministerial advice; (b) the right to encourage a given policy or course of administrative action; and (c) the right to warn the Cabinet against the same. In these respects, the emperor personifies the democratic state, sanctions legitimate authority, ensures the legality of his official acts, and guarantees the execution of the public will. These functions, when considered altogether, serve two purposes: foremost, to uphold the continuity and stability of Japanese democracy; and second, to foster a shared national identity and cultural heritage that transcends party politics. In order to maintain his institutional neutrality as Japan's national symbol, he is barred from making political statements.

It is the emperor's preeminent constitutional duty to appoint the prime minister as designated by the Diet and the chief justice as designated by the Cabinet. However, the emperor does not have the authority to decline the nominations. The emperor's other responsibilities, laid down in Article 7 of the Constitution, concern the basic functioning of the state. To this end, the emperor, on behalf of the Japanese people:

Regular ceremonies of the emperor with a constitutional basis are the Imperial Investitures (Shinninshiki) in the Tokyo Imperial Palace and the Speech from the Throne ceremony in the House of Councillors in the National Diet Building. The latter ceremony opens ordinary and extra sessions of the Diet. Ordinary sessions are opened each January and also after new elections to the House of Representatives. Extra sessions usually convene in the autumn and are opened then.^{[non-primary source needed]}