The Holy Roman Emperor (German: Römisch-Deutscher Kaiser, or "Roman-German Emperor" Latin: Imperator Romanus Sacer) is a term used by historians to denote a medieval ruler who had also received the title of "Emperor of the Romans" from the Pope. After the 16th century, this elected monarch governed the Holy Roman Empire (later called Holy Roman Empire of the German Nation), a Central European union of territories of the Medieval and Early Modern period.

The title of Emperor (Imperator) carried with it an important role as protector of the Catholic Church. As the papacy's power grew during the Middle Ages, Popes and emperors came into conflict over church administration. The best-known and bitterest conflict was that known as the Investiture Controversy, fought during the 11th century between Henry IV and Pope Gregory VII.

After Charlemagne was crowned Roman Emperor by the Pope, his successors maintained the title until the death of Berengar I of Italy in 924. No pope appointed an emperor again until the coronation of Otto the Great in 962. Otto is considered the first Holy Roman Emperor, although Charlemagne is also accounted by some to be the first. Under Otto and his successors, much of the former Carolingian kingdom of Eastern Francia became the Holy Roman Empire. The various German princes elected one of their peers as King of the Germans, after which he would be crowned as emperor by the Pope. After Charles V's coronation, all succeeding emperors were legally emperors-elect due to the lack of papal coronation, but for all practical purposes they were simply called emperors.

The Roman emperor was the ruler of the Roman State during the imperial period (starting at about 27 BC). The Romans had no single term for the office although at any given time, a given title was associated with the emperor. If a man was "proclaimed emperor" this normally meant he was proclaimed augustus, or (for generals) imperator (from which English emperor ultimately derives). Several other titles and offices were regularly accumulated by emperors, such as caesar, princeps senatus, consul and Pontifex Maximus. The power of emperors was generally based on the accumulation of powers from republican offices and the support of the army.

Roman emperors refused to be considered "kings", instead claiming to be leaders of a republic, however nominal. The first emperor, Augustus, resolutely refused recognition as a monarch. Although Augustus could claim that his power was authentically Republican, his successor, Tiberius, could not convincingly make the same claim. Nonetheless, the Republican institutional framework (senate, consuls, magistracies etc.) was preserved until the very end of the Western Empire.

The Holy Roman Empire (HRE; German: Heiliges Römisches Reich (HRR), Latin: Imperium Romanum Sacrum (IRS), Italian: Sacro Romano Impero (SRI)) was a German empire that existed from 962 to 1806 in Central Europe. It was ruled by the Holy Roman Emperor. Its character changed during the Middle Ages and the Early Modern period, when the power of the emperor gradually weakened in favour of the princes. In its last centuries, its character became quite close to a union of territories.

The empire's territory was centered on the Kingdom of Germany, and included neighboring territories, which at its peak included the Kingdom of Italy and the Kingdom of Burgundy. For much of its history, the Empire consisted of hundreds of smaller sub-units, principalities, duchies, counties, Free Imperial Cities and other domains.

In 962 Otto I was crowned Holy Roman Emperor (German: Römisch-Deutscher Kaiser), although the Roman imperial title was first restored to Charlemagne in 800. Otto was the first emperor of the realm who was not a member of the earlier Carolingian dynasty. The last Holy Roman Emperor was Francis II, who abdicated and dissolved the Empire in 1806 during the Napoleonic Wars. In a decree following the 1512 Diet of Cologne, the name was officially changed to Holy Roman Empire of the German Nation (German: Heiliges Römisches Reich Deutscher Nation, Latin: Imperium Romanum Sacrum Nationis Germanicæ).