Ferdinand I (Ferdinand Maximilian Karl Leopold Maria; 26 February 1861 – 10 September 1948) was Prince of Bulgaria from 1887 to 1908 and Tsar of Bulgaria from 1908 until his abdication in 1918. Under his rule, Bulgaria entered the First World War on the side of the Central Powers in 1915.

Ferdinand was born on 26 February 1861 in Vienna, a German prince of the House of Saxe-Coburg and Gotha-Koháry. He was the son of Prince August of Saxe-Coburg and his wife Clémentine of Orléans, daughter of King Louis Philippe I of France. Princess Maria Antonia Koháry was a Hungarian Noble and heiress who married Ferdinand's grandfather, Prince Ferdinand of Saxe-Coburg and Gotha. Ferdinand was raised in his parents’ Catholic faith and baptised in St. Stephen's Cathedral, Vienna on 27 February, having as godparents Archduke Maximilian of Austria and his wife Princess Charlotte of Belgium. He grew up in the cosmopolitan environment of Austro-Hungarian high nobility and also in their ancestral lands in Hungary and in Germany. The House of Koháry descended from an immensely wealthy Upper Hungarian noble family, who held the princely lands of Čabraď and Sitno in present-day Slovakia, among others. The family's property was augmented by Clémentine of Orléans' remarkable dowry.

Ferdinand was a grandnephew of Ernest I, Duke of Saxe-Coburg and Gotha and of Leopold I, first king of the Belgians. His father August was a brother of King Ferdinand II of Portugal, and also a first cousin to Queen Victoria, her husband Albert, Empress Carlota of Mexico and her brother Leopold II of Belgium. These last two, Leopold and Carlota, were also first cousins of Ferdinand I's through his mother, a princess of Orléans. This made the Belgian siblings his first cousins, as well as his first cousins once removed. Indeed, the House of Saxe-Coburg and Gotha had contrived to occupy, either by marriage or by direct election, several European thrones in the course of the 19th century. Following the family trend, Ferdinand was himself to found the royal dynasty of Bulgaria.

The previous ruling prince of Bulgaria, Alexander of Battenberg, had abdicated in 1886 after a pro-Russian coup, only seven years after he had been elected. Ferdinand, who was an officer in the Austro-Hungarian Army, was elected Prince of autonomous Bulgaria by its Grand National Assembly on 7 July 1887 in the Gregorian calendar (the "New Style" used hereinafter). The throne had been previously offered, before Ferdinand's acceptance, to princes from Denmark to the Caucasus and even to the King of Romania. The Russian tsar himself had nominated his aide, Nichols Dadian of Mingrelia, but his candidacy was rejected by the Bulgarians. Ferdinand's accession was greeted with disbelief in many of the royal houses of Europe; Queen Victoria, his father's first cousin, stated to her prime minister, "He is totally unfit ... delicate, eccentric and effeminate ... Should be stopped at once." To the amazement of his initial detractors, Ferdinand generally gave a good account of himself during the first two decades of his reign.

Bulgaria's domestic political life was dominated during the early years of Ferdinand's reign by liberal party leader Stefan Stambolov, whose foreign policy saw a marked cooling in relations with Russia, formerly seen as Bulgaria's protector.

Stambolov's fall (May 1894) and subsequent assassination (July 1895) — probably not engineered by Ferdinand — paved the way for a reconciliation of Bulgaria with Russia, effected in February 1896 with Ferdinand's decision to convert his infant son, Prince Boris, from Roman Catholicism to Eastern Orthodoxy. However, this earned him the animosity of his Catholic Austrian relatives, and he was excommunicated by Pope Leo XIII.

On 5 October 1908 (celebrated on 22 September), Ferdinand proclaimed Bulgaria's de jure independence from the Ottoman Empire (though the country had been de facto independent since 1878). He also proclaimed Bulgaria a kingdom, and assumed the title of tsar—a deliberate nod to the rulers of the earlier Bulgarian states. However, while the title tsar was translated as "emperor" in the First and Second Bulgarian empires, it was translated as "king" under Ferdinand and his successors. The Bulgarian Declaration of Independence was proclaimed by him at the Holy Forty Martyrs Church in Tarnovo, and was recognized by the Ottoman Empire and the other European powers. The Tarnovo Constitution was retained, with the word "prince" replaced by the word "tsar."

On a visit to German Emperor Wilhelm II, his second cousin once removed, in 1909, Ferdinand was leaning out of a window of the New Palace in Potsdam when the Emperor came up behind him and slapped him on the bottom. Ferdinand was affronted by the gesture, but the Kaiser refused to apologize. Ferdinand however exacted his revenge by awarding a valuable arms contract he had intended to give to the Krupp's factory in Essen to French arms manufacturer Schneider-Creusot. Another incident occurred on his journey to the funeral of his second cousin King Edward VII of the United Kingdom in 1910. A tussle broke out over where his private railway carriage would be positioned in relation to the heir presumptive to the Austro-Hungarian throne, Archduke Franz Ferdinand. The Archduke won out, having his carriage positioned directly behind the engine. Ferdinand's was placed directly behind his. Realising the dining car of the train was behind his own carriage, Ferdinand obtained his revenge on the Archduke by refusing him passage through his own carriage to the dining car. On 15 July the same year during a visit to Belgium, Ferdinand also became the first head of state to fly in an airplane. He awarded the pilot of the plane with a medal when they landed.