The Imperial Diet (German: Reichstag; Latin: Dieta Imperii or Comitium Imperiale) was the deliberative body of the Holy Roman Empire. It was not a legislative body in the contemporary sense; its members envisioned it more like a central forum where it was more important to negotiate than to decide.

Its members were the Imperial Estates, divided into three colleges. The diet as a permanent, regularized institution evolved from the Hoftage (court assemblies) of the Middle Ages. From 1663 until the end of the empire in 1806, it was in permanent session at Regensburg.

All Imperial Estates enjoyed immediacy and, therefore, they had no authority above them besides the Holy Roman Emperor himself. While all the estates were entitled to a seat and vote, only the higher temporal and spiritual princes of the College of Princes enjoyed an individual vote (Virilstimme), while lesser estates such as imperial counts and imperial abbots, were merely entitled to a collective vote (Kuriatstimme) within their particular bench (Curia), as did the free imperial cities belonging to the College of Towns.

The right to vote rested essentially on a territorial entitlement, with the result that when a given prince acquired new territories through inheritance or otherwise, he also acquired their voting rights in the diet. In general, members did not attend the permanent diet at Regensburg, but sent representatives instead. The late imperial diet was in effect a permanent meeting of ambassadors between the estates.

The role and function of the Imperial Diet evolved over the centuries, like the Empire itself, with the estates and separate territories increasing control of their own affairs at the expense of imperial power. Initially, there was neither a fixed time nor location for the Diet. It began as a convention of the dukes of the old Germanic tribes that formed the Frankish kingdom when important decisions had to be made, probably based on the old Germanic law whereby each leader relied on the support of his leading men. In the early and high Middle Ages these assemblies were not yet institutionalized, but were held as needed at the decision of the king or emperor. They weren't called Diet yet, but Hoftag (court day). They were usually held in the imperial palaces (Kaiserpfalz).

For example, already under Charlemagne during the Saxon Wars, a Hoftag, according to the Royal Frankish Annals, met at Paderborn in 777 and determined laws over the subdued Saxons and other tribes. In 803 Charlemagne, by then crowned as emperor of the Franks, issued the final version of the Lex Saxonum.

At the Diet of 919 in Fritzlar the dukes elected the first King of the Germans, who was a Saxon, Henry the Fowler, thus overcoming the longstanding rivalry between Franks and Saxons and laying the foundation for the German realm. After the conquest of Italy, the 1158 Diet of Roncaglia finalized four laws that would significantly alter the (never formally written) constitution of the Empire, marking the beginning of the steady decline of the central power in favour of the local dukes. The Golden Bull of 1356 cemented the concept of "territorial rule" (Landesherrschaft), the largely independent rule of the dukes over their respective territories, and also limited the number of electors to seven. The Pope, contrary to modern myth, was never involved in the electoral process but only in the process of ratification and coronation of whomever the Prince-Electors chose.

Until the late 15th century the Diet was not formalized as an institution. Instead, the dukes and other princes would irregularly convene at the court of the Emperor. These assemblies were usually referred to as Hoftage (from German Hof "court"). Only beginning in 1489 was the Diet called the Reichstag, and it was formally divided into collegia ("colleges").