Hungarians constitute the largest minority in Slovakia. According to the 2021 Slovak census, 422,065 people (or 7.75% of the population) declared themselves Hungarian. An additional 34,089 people declared a combination of Hungarian and other nationalities. Altogether 456,154 people stated that Hungarian was their mother tongue.

Hungarians in Slovakia are predominantly concentrated in the southern part of the country, near the border with Hungary. They form the majority in two districts, Komárno and Dunajská Streda.

After the defeat of the Central Powers on the Western Front in 1918, the Treaty of Trianon was signed between the winning Entente powers and Hungary in 1920 at the Paris Peace Conference. The treaty greatly reduced the Kingdom of Hungary's borders, including ceding all of Upper Hungary to Czechoslovakia, in which Slovaks made up the dominant ethnicity. In consideration of the strategic and economic interests of their new ally, Czechoslovakia, the victorious allies set the Czechoslovak–Hungarian border further south than the Slovak–Hungarian language border. Consequently, the newly created state contained areas that were overwhelmingly ethnic Hungarian.

According to the 1910 census conducted in Austria-Hungary, there were 884,309 ethnic Hungarians, constituting 30.2% of the population in what is now Slovakia and Carpatho-Ukraine. The Czechoslovak census of 1930 recorded 571,952 Hungarians. In the 2001 census, by contrast, the percentage of ethnic Hungarians in Slovakia was 9.7%, a decrease of two-thirds in percentage but not in absolute number, which remained roughly the same.

Between 1880 and 1910, the Hungarian population increased by 55.9%, while Slovak population increased by only 5.5% though Slovaks had a higher birth rate at the same time. The level of differences does not explain this process by emigration (higher among Slovaks) or by population moves and natural assimilation during industrialization. In 16 northern counties, the Hungarian population rose by 427,238, while the majority Slovak population rose only by 95,603. The number of "Hungarians who can speak Slovak" unusually increased in a time when Hungarians really had no motivation to learn it – by 103,445 in southern Slovakia in absolute numbers, by 100% in Pozsony, Nyitra, Komárom, Bars and Zemplén County and more than 3 times in Košice. After the creation of Czechoslovakia, people could declare their nationality more freely.

Furthermore, censuses from the Kingdom of Hungary and Czechoslovakia differed in their view on the nationality of the Jewish population. Czechoslovakia allowed Jews to declare a separate Jewish nationality, while Jews were counted mostly as Hungarians in the past. In 1921, 70,529 people declared Jewish nationality.

The population of larger towns like Košice or Bratislava were historically bilingual or trilingual, and some might declare the most-popular or the most-beneficial nationality at a particular time. According to the Czechoslovak censuses, 15–20% of the population in Košice was Hungarian, but during the parliamentary elections, the "ethnic" Hungarian parties received 35–45% of the total votes (excluding those Hungarians who voted for the Communists or the Social Democrats). However, such comparisons are not fully reliable, because "ethnic" Hungarian parties did not necessarily present themselves to Slovak population as "ethnic", and also had Slovak subsidiaries.

Hungarian state employees who refused to take an oath of allegiance had to decide between retirement and moving to Hungary. The same applied to Hungarians who did not receive Czechoslovak citizenship, who were forced to leave or simply did not self-identify with the new state. Two examples of people forced to leave were the families of Béla Hamvas and Albert Szent-Györgyi. The numerous refugees (including even more from Romania) necessitated the construction of new housing projects in Budapest (Mária-Valéria telep, Pongrácz-telep), which gave shelter to refugees numbering at least in the tens of thousands.