Ferdinand I (Ferdinand Viktor Albert Meinrad; 24 August 1865 – 20 July 1927), nicknamed Întregitorul ("the Unifier"), was King of Romania from 10 October 1914 until his death in 1927. Ferdinand was the second son of Leopold, Prince of Hohenzollern, and Infanta Antónia of Portugal, (daughter of Queen Maria II of Portugal and of Prince Ferdinand of Saxe-Coburg and Gotha-Kohary). His family was part of the Catholic branch of the Prussian royal family Hohenzollern.

In 1886, Ferdinand became heir presumptive to the Romanian throne, following the renunciation of his father (in 1880) and older brother. From the moment he settled in Romania, he continued his military career, gaining a series of honorary commands and being promoted to the rank of corps general. He married in 1893 Princess Marie of Edinburgh, granddaughter of both Queen Victoria and Emperor Alexander II.

Ferdinand became King of Romania on 10 October 1914, under the name Ferdinand I, following the death of his uncle, King Carol I. He ruled Romania during World War I, choosing to side with the Triple Entente against the Central Powers. This led to Kaiser Wilhelm II of Germany removing his name from the royal house of Hohenzollern. At the war's end, Romania emerged as a much-enlarged kingdom due to Bessarabia, Bukovina and Transylvania and parts of Banat, Crișana, and Maramureș becoming part of the Kingdom of Romania in 1920, and Ferdinand was crowned king of "Greater Romania" in a grand ceremony in 1922. In the years following the establishment of Greater Romania, Romanian society went through a series of major transformations, especially to the application of the agrarian reform and of the universal vote. In 1925, his eldest son, Prince Carol, gave up the rights of succession to the royal crown of Romania leading to a dynastic crisis, as the next prince in line of succession was Carol's 4-year-old son, Prince Michael. This led Ferdinand to remove Prince Carol's name from the royal house of Romania.

Ferdinand died from cancer in 1927 and was succeeded by his grandson Michael under a regency formed by three people: Prince Nicholas of Romania, the younger brother of Prince Carol; patriarch Miron Cristea; and president of the Supreme Court of Justice Gheorghe Buzdugan.

Prince Ferdinand Viktor Albert Meinrad of Hohenzollern-Sigmaringen was born in Sigmaringen in southwestern Germany. The name was later shortened simply to Hohenzollern after the extinction of the Hohenzollern-Hechingen branch in 1869. The princes of Hohenzollern-Sigmaringen had ruled the principality until 1850, when it was annexed to Prussia.

Ferdinand I was the son of Leopold, Prince of Hohenzollern-Sigmaringen, and Infanta Antónia of Portugal (1845–1913), daughter of Queen Maria II of Portugal and Prince Ferdinand of Saxe-Coburg and Gotha, heir to the Hungarian magnates of Koháry on his mother's side.

Following the renunciations, first of his father in 1880 and then of his elder brother Prince Wilhelm of Hohenzollern-Sigmaringen in 1886, young Ferdinand became the heir-presumptive to the throne of his childless uncle, King Carol I of Romania, who reigned until his death in October 1914. In 1889, the Romanian parliament recognized Ferdinand as a prince of Romania. The Romanian government did not require his conversion to Eastern Orthodoxy from Catholicism, as was the common practice prior to this date, thus allowing him to continue with his born creed, but it was required that his children be raised Orthodox, the state religion of Romania. For agreeing to this, Ferdinand was excommunicated from the Catholic Church, although this was later lifted.

Ferdinand's mother's first cousin Tsar Ferdinand I of Bulgaria sat on the throne of the neighbouring Bulgaria beginning in 1887, and was to become the greatest opponent of the kingdom of his Romanian cousins. The neighboring Emperor Francis Joseph, monarch of Austria-Hungary and as such, ruler of Transylvania, was Ferdinand's grandmother's first cousin.