The Constitutional Democratic Party (Russian: Конституцио́нно-демократи́ческая па́ртия, romanized: Konstitutsionno-demokraticheskaya partiya, K-D), also called Constitutional Democrats and formally the Party of People's Freedom (Russian: Па́ртия Наро́дной Свобо́ды), was a political party in the Russian Empire that promoted Western constitutional monarchy—among other policies—and attracted a base ranging from moderate conservatives to mild socialists. Party members were called Kadets (or Cadets) from the abbreviation K-D of the party name. Konstantin Kavelin's and Boris Chicherin's writings formed the theoretical basis of the party's platform. Historian Pavel Miliukov was the party's leader throughout its existence.

The Kadets' base of support were primarily intellectuals and professionals; university professors and lawyers were particularly prominent within the party. Many Kadet party members were veterans of the zemstvo, local councils. The Constitutional Democratic Party formed from the merger of several liberal groupings, namely the Union of Liberation, the Union of Zemstvo Constitutionalists and the Union of Unions as well as the organization of bourgeois professionals and intellectuals, including teachers, lawyers, writers, physicians and engineers.

The Kadets' liberal economic program favored the workers' right to an eight-hour day and the right to take strike action. The Kadets "were unwaveringly committed to full citizenship for all of Russia's minorities" and supported Jewish emancipation. The party drew significant support from Jews until 1916, and Volga Germans and a significant number of each group were active party members. On the other hand, the Kadets adhered to Russian nationalism as they largely based their identity on "Russian nation" or the "Russian people" as something opposed to the state bureaucracy; since 1905, they drifted towards statism, and their views on foreign politics were based on the view of international politics as a "national struggle", and they generally advocated for Russian imperialism, describing Russians as Staatsvolk, and the Russian Empire as their nation-state. However, they differed from the hardline ethnocentric Russian nationalists, as they understood Russians rather as a political identity and defended the rights of ethnic minorities and nations of Russia to have cultural authonomies and to enter the Russian nation. Such views and Pan-Slavism, which they shared with the other moderate right-wing parties, drove them into a very hostile attitude towards Germany and Austria-Hungary during World War I, and by 1917, they were strongly nationalist and defensist; during the Russian Civil War, they became proponents of military dictatorship and territorial integrity of the Russian Empire, and were the strongest supporters of the Whites next to the nationalist parties.

The Constitutional Democratic Party was formed in Moscow on 12–18 October 1905 at the height of the Russian Revolution of 1905 when Tsar Nicholas II was forced to sign the October Manifesto granting basic civil liberties. The Kadets were to the immediate left of the Octobrists, another new formed party organized at the same time. Unlike the Octobrists, who were committed to constitutional monarchy from the start, the Kadets were at first ambiguous on the subject, demanding universal suffrage (including women's suffrage) and a Constituent Assembly that would determine the country's form of government. This radicalism was despite the fact 60% of Kadets were nobles. The Kadets were one of the parties invited by the reform-minded Prime Minister Sergei Witte to join his cabinet in October–November 1905, but the negotiations broke down over the Kadets' radical demands and Witte's refusal to drop notorious reactionaries like Petr Nikolayevich Durnovo from the cabinet.

With some socialist and revolutionary parties boycotting the election to the First Duma in February 1906, the Kadets received 37% of the urban vote and won over 30% of the seats in the Duma. They interpreted their electoral win as a mandate and allied with the left-leaning peasant Trudovik faction, forming a majority in the Duma. When their declaration of legislative intent was rejected by the government at the start of the parliamentary session in April, they adopted a radical oppositionist line, denouncing the government at every opportunity. On 9 July, the government announced that the Duma was dysfunctional and dissolved it. In response, 120 Kadet and 80 Trudovik and Social Democrat deputies went to in Vyborg, Finland (and thus beyond the reach of Russian police) and responded with the Vyborg Manifesto (or the "Vyborg Appeal"), written by Miliukov. In the manifesto, they called for passive resistance, non-payment of taxes and draft avoidance. The appeal failed to have an effect on the population at large and proved both ineffective and counterproductive, leading to a ban on its authors, including the entire Kadet leadership, from participation in future Dumas. This was further accentuated by the force of the tsar trying to control and deteriorate the power of the Duma.

It was not until later in 1906, with the revolution in retreat, that the Kadets abandoned revolutionary and republican aspirations and declared their support for a constitutional monarchy. The government remained suspicious of the Kadets until the fall of the monarchy in 1917. Finnish liberal politician and professor of jurisdiction and politology Leo Mechelin was expelled 1903–1904 when the Kadets were preparing to form a party. Mechelin cooperated with them and wrote them a liberal constitution for Russia to be enforced when they would get into power. At the time of Vyborg Manifesto, Mechelin was already the leader of the Finnish government, or "Mechelin's senate" (1905–1908), which implemented the universal right to vote and freedoms of expression, press, congregation and association.

When the Second Duma was convened on 20 February 1907, the Kadets found themselves in a difficult position. Their leadership was not represented in the Duma after the Vyborg Manifesto fiasco and their numbers were reduced to about 100. Although still the largest faction in the Duma, they no longer dominated the parliament and their attempts to concentrate on lawmaking were frustrated by radicals on the left and on the right who saw the Duma as a propaganda tool. Although the Kadets had moderated their position in the Second Duma, they refused to vote in May 1907 for a resolution denouncing revolutionary violence which gave the government of Pyotr Stolypin a pretext to dissolve the Second Duma on 3 June 1907 and change the electoral law to drastically limit the representation of leftist and liberal parties.

Due to the changes in the electoral law, the Kadets were reduced to a relatively small (54 seats) opposition group in the Third Duma (1907–1912). Although excluded from the more important Duma committees, the Kadets were not entirely powerless and could determine the outcome of certain votes when allied with the centrist Octobrist faction against right-wing nationalist deputies. With the revolution crushed by 1908, they moderated their position even further as they voted to denounce revolutionary violence, no longer sought confrontation with the government and concentrated on influencing legislation whenever possible. By 1909, Miliukov could claim that the Kadets were now "the opposition of His Majesty, not the opposition to His Majesty", which caused only moderate dissent among the left-leaning faction of the party.