The Reichstag (German: [ˈʁaɪçstaːk] ^ⓘ, "Diet of the Realm"), of the German Empire was Germany's lower House of Parliament from 1871 to 1918. Within the governmental structure of the Reich, it represented the national and democratic element alongside the federalism of the Bundesrat and the monarchic and bureaucratic element of the executive, embodied in the Reich chancellor. Together with the Bundesrat, the Reichstag had legislative power and shared in decision-making on the budget. It also had certain rights of control over the executive branch and could engage the public through its debates. The emperor had little political power, and over time the position of the Reichstag strengthened with respect to both the imperial government and the Bundesrat.

Reichstag members were elected for three-year terms from 1871 to 1888 and following that for five years. It had one of the most progressive electoral laws of its time: with only a few restrictions, all male citizens 25 and older were allowed to vote, secretly and equally, in direct elections. The Reichstag met throughout the First World War, but was prevented from sitting during the German Revolution of 1918–1919. Its last session took place on 26 October 1918. Its successors were the Weimar National Assembly (February 1919 to June 1920), followed by the Reichstag of the Weimar Republic that met for the first time in June of 1920.

The 1871 Constitution of the German Empire did not change the legal form of the parliament as it had been laid down for the Reichstag of the North German Confederation in its 1867 constitution.

Members were elected by universal, equal and secret manhood suffrage, with the voting age set at 25. The suffrage was quite extensive both in comparison to other countries and to the various German state parliaments. In most countries at that time, votes were not equal but weighted by such factors as class or income (census suffrage). Prussia, for example, the largest state in the Empire, used a three-class franchise in elections to the Prussian House of Representatives, with votes weighted by the amount of tax paid.

Since it was thought that allowing men in active military service to vote would politicize the military, they were denied the franchise, although they had the right to stand for election. Also ineligible to vote were men dependent on public assistance for the poor, those over whose assets bankruptcy or insolvency proceedings had been initiated, and those who had been declared incapacitated or been deprived of their civil rights by a court judgment. In the 1912 Reichstag elections, 22.2% of the population (14.442 million men) were eligible to vote; by comparison the figures were 16% in Great Britain and 28% in the United States. The percentage eligible to vote in Reichstag elections was also significantly higher than in German state elections, such as in Prussia, Bavaria and Saxony, where the right to vote was subject to additional conditions.

Elections were held in districts that elected a single member by absolute majority, which meant that – unlike under the proportional representation of the Weimar Republic and the Federal Republic of Germany – there were only directly elected deputies. If no candidate received an absolute majority in the first round, a runoff election was held between the two candidates with the highest number of votes. Runoff elections became increasingly important during the life of the Empire. Whereas in the 1874 Reichstag election, runoffs had to be held in 46 of the 397 electoral districts (11.6%), in the 1890 election there were 147 (37%) and 190 in 1912 (47.9%).

In 1871 the Reichstag consisted of 382 deputies. After the addition of fifteen electoral districts to Alsace–Lorraine in 1874, there were 397 until the end of the Empire in 1918. The electoral districts were initially drawn to include about 100,000 people each, with the exception of eight small states that formed their own electoral districts even though they had fewer than 100,000 inhabitants. Since the electoral district boundaries were based on the borders of the individual German states, some electoral districts consisted of widely separated areas. The fragmentation was particularly pronounced in the Thuringian states.

Due to differing rates of population growth caused primarily by internal migration to the big cities and industrial centers, large differences arose in the populations of the individual electoral districts. In 1912 there were twelve electoral districts with fewer than 75,000 inhabitants and twelve with more than 400,000, the largest of which, electoral district 10 in Potsdam, had 1,282,000. The layout of the electoral districts, which was based on the 1864 census and did not change afterwards, disadvantaged those political parties that had their constituencies primarily in the cities. The small electoral districts that coincided with individual "dwarf" states continued to send one deputy to the Reichstag because the federal structure of the Reich required that each state have a seat regardless of population, as was notably the case in Schaumburg-Lippe, with a population of about 45,000 in 1912.