Boris III (Bulgarian: Борѝс III; Boris Treti; 30 January [O.S. 18 January] 1894 – 28 August 1943) was the Tsar of the Kingdom of Bulgaria from 1918 until his death in 1943.

The eldest son of Ferdinand I, Boris assumed the throne upon the abdication of his father in the wake of Bulgaria's defeat in World War I. Under the 1919 Treaty of Neuilly, Bulgaria was forced to cede various territories, pay crippling war reparations, and greatly reduce the size of its military. That same year, Aleksandar Stamboliyski of the Bulgarian Agrarian National Union became prime minister. After Stamboliyski was overthrown in a coup in 1923, Boris recognized the new government of Aleksandar Tsankov, who harshly suppressed the Bulgarian Communist Party and led the nation through a brief border war with Greece. Tsankov was removed from power in 1926, and a series of prime ministers followed until 1934, when the corporatist Zveno (Bulgarian: Звено) movement staged a coup and outlawed all political parties. Boris opposed the Zveno government and overthrew them in 1935, eventually installing Georgi Kyoseivanov as prime minister. For the remainder of his reign, Boris would rule as a de facto absolute monarch, with his prime ministers largely submitting to his will.

Following the outbreak of World War II, Bulgaria initially remained neutral. In 1940, Nazi sympathizer Bogdan Filov replaced Kyoseivanov as prime minister, becoming the last prime minister to serve under Boris. In September 1940, with the support of Nazi Germany, Bulgaria received the region of Southern Dobrudja from Romania as part of the Treaty of Craiova. In January 1941, Boris approved the anti-Semitic Law for Protection of the Nation, which denied citizenship to Bulgarian Jews and placed numerous restrictions upon them. In March 1941, Bulgaria joined the Axis and allowed German troops to use Bulgaria as a base from which to invade Yugoslavia and Greece. Bulgaria then received large portions of Yugoslav Macedonia, Pirot County in eastern Serbia and Greek Thrace, which were key targets of Bulgarian irredentism. Bulgaria opted out of participation in the German invasion of the Soviet Union, as allowed by the provisions of the Axis alliance. As part of the Holocaust, Bulgarian authorities deported most Jews from occupied Greek and Yugoslav territories and transferred them to the German extermination camp of Treblinka. Under public pressure, Boris cancelled the deportation of Bulgarian Jews while expelling almost 20,000 Jews to the Bulgarian countryside to be deployed in forced labour camps. In 1942, Zveno, the Agrarian National Union, the Bulgarian Communist Party, and other far-left groups united to form a resistance movement known as the Fatherland Front, which went on to overthrow the government in 1944. In August 1943, shortly after returning from a visit to Germany, Boris died at the age of 49. Following his death, he was succeeded as Tsar by his six-year-old son, Simeon Borisov Saxe-Coburg-Gotha (Bulgarian: Симеон Борисов Сакскобургготски), who ascended the throne as Simeon II.

Boris was born on 30 January 1894 in Sofia to Ferdinand I, Prince of Bulgaria, and his wife Princess Marie Louise of Bourbon-Parma.

In February 1896, his father paved the way for the reconciliation of Bulgaria and Russia with the conversion of the infant Prince Boris from Catholicism to Eastern Orthodoxy, a move that earned Ferdinand the frustration of his wife, the animosity of his Catholic Austrian relatives (particularly his uncle Franz Joseph I of Austria) and excommunication by Pope Leo XIII. In order to remedy this difficult situation, Ferdinand had his subsequent children baptised in the Catholic Church. Nicholas II of Russia stood as godfather to Boris and later met the young boy during Ferdinand's official visit to Saint Petersburg in July 1898.

He received his initial education in the so-called Palace Secondary School, which Ferdinand had founded in 1908 solely for his sons. Later, Boris graduated from the Military School in Sofia, then took part in the Balkan Wars. During the First World War, he served as liaison officer of the General Staff of the Bulgarian Army on the Macedonian front. In 1916, he was promoted to colonel and attached again as liaison officer to Army Group Mackensen and the Bulgarian Third Army for the operations against Romania. Boris worked hard to smooth the sometimes difficult relations between Field Marshal Mackensen and Lieutenant General Stefan Toshev, the commander of the Third Army. Through his courage and personal example, he earned the respect of the troops and the senior Bulgarian and German commanders, even that of the Generalquartiermeister of the German Army, Erich Ludendorff, who preferred dealing personally with Boris and described him as excellently trained, a thoroughly soldierly person and mature beyond his years. In 1918, Boris was made a major general.

In September 1918, Bulgaria was defeated in the Vardar Offensive and forced to sue for peace. Ferdinand abdicated in favour of Boris, who became Tsar on 3 October 1918.

A year after Boris's accession, Aleksandar Stamboliyski (also Stambolijski) of the Bulgarian Agrarian National Union was elected prime minister. Though popular with the large peasant class, Stambolijski earned the animosity of the middle class and military, which led to his toppling in a military coup on 9 June 1923 and assassination. On 14 April 1925, an anarchist group attacked Boris's cavalcade as it passed through the Arabakonak Pass. Two days later, a bomb killed 213 members of the Bulgarian political and military elite in Sofia as they attended the funeral of a murdered general in the Saint Nedelya Church terror assault. Following a further attempt on Boris's life the same year, military reprisals killed several thousand communists and agrarians, including representatives of the intelligentsia. Finally, in October 1925, there was a short border war with Greece, known as the Petrich Incident, which was resolved with the help of the League of Nations.