Sofia is the capital and largest city of Bulgaria. It is situated in the Sofia Valley at the foot of the Vitosha mountain, in the western part of the country. The city is built west of the Iskar river and has many mineral springs, such as the Sofia Central Mineral Baths. It has a humid continental climate.

Known as Serdica in antiquity, Sofia has been an area of human habitation since at least 7000 BC. The recorded history of the city begins with the attestation of the conquest of Serdica by the Roman Republic in 29 BC from the Celtic tribe Serdi. During the decline of the Roman Empire, the city was raided by Huns, Visigoths, Avars, and Slavs. In 809, Serdica was incorporated into the First Bulgarian Empire by Khan Krum and became known as Sredets. In 1018, the Byzantines ended Bulgarian rule until 1194, when it was reincorporated by the Second Bulgarian Empire. Sredets became a major administrative, economic, cultural and literary hub until its conquest by the Ottomans in 1382. From 1530 to 1836, Sofia was the regional capital of Rumelia Eyalet, the Ottoman Empire's largest and most important province. Bulgarian rule was restored in 1878. Sofia was selected as the capital of the Third Bulgarian State in the next year, ushering a period of intense demographic and economic growth.

Sofia is the 14th-largest city in the European Union. It is surrounded by mountains such as Vitosha to the south, Lyulin to the west, and the Balkan Mountains to the north. It is the third highest European capital after Andorra la Vella and Madrid. Sofia is home to several universities, cultural institutions and commercial companies. The city has been described as the "triangle of religious tolerance". This is because three temples of three major world religions—Christianity, Islam and Judaism—are situated close together: Sveta Nedelya Church, Banya Bashi Mosque and Sofia Synagogue. This triangle was recently expanded to a "square" and includes the Catholic Cathedral of St Joseph.

The Boyana Church in Sofia, constructed during the Second Bulgarian Empire and holding much patrimonial symbolism to the Bulgarian Orthodox Church, was included onto the World Heritage List in 1979. With its cultural significance in Southeast Europe, Sofia is home to the National Opera and Ballet of Bulgaria, the National Palace of Culture, the Vasil Levski National Stadium, the Ivan Vazov National Theatre, the National Archaeological Museum, and the Serdica Amphitheatre. The Museum of Socialist Art includes many sculptures and posters that educate visitors about the lifestyle in communist Bulgaria.

The population of Sofia declined from 70,000 in the late 18th century, through 19,000 in 1870, to 11,649 in 1878, after which it began increasing. Sofia hosts some 1.28 million residents within a territory of 500 km², a concentration of 17.9% of the country's population within the 200th percentile of the country's territory. The urban area of Sofia hosts some 1.5 million residents within 5723 km², which comprises Sofia City Province and parts of Sofia Province (Dragoman, Slivnitsa, Kostinbrod, Bozhurishte, Svoge, Elin Pelin, Gorna Malina, Ihtiman, Kostenets) and Pernik Province (Pernik, Radomir), representing 5.16% of the country territory. The metropolitan area of Sofia is based upon one hour of car travel time, stretches internationally and includes Dimitrovgrad in Serbia. The metropolitan region of Sofia is inhabited by a population of 1.6 million.

For a long time, the city possessed the name, Serdica (Ancient Greek: Σαρδική, Serdikē, or Σαρδική, Sardikē; Latin: Serdica or Sardica), derived from the tribe Serdi, who were either of Thracian, Celtic, or mixed Thracian-Celtic origin. The emperor Marcus Ulpius Traianus (53–117 AD) gave the city the combinative name of Ulpia Serdica; Ulpia may be derived from an Umbrian cognate of the Latin word lupus, meaning "wolf" or from the Latin vulpes (fox). It seems that the first written mention of Serdica was made during his reign and the last mention was in the 19th century in a Bulgarian text (Сардакіи, Sardaki). Other names given to Sofia, such as Serdonpolis (Byzantine Ancient Greek: Σερδών πόλις, "City of the Serdi") and Triaditza (Τριάδιτζα, "Trinity"), were mentioned by Byzantine Greek sources or coins. The Slavic name Sredets (Church Slavonic: Срѣдецъ), which is related to "middle" (среда, "sreda") and to the city's earliest name, first appeared on paper in an 11th-century text. The city was called Atralisa by the Arab traveller Idrisi and Strelisa, Stralitsa, or Stralitsion by the Crusaders.

The name Sofia comes from the Saint Sofia Church, as opposed to the prevailing Slavic origin of Bulgarian cities and towns. The origin is in the Greek word sophía (σοφία, "wisdom"). The earliest works where this latest name is registered are the duplicate of the Gospel of Serdica, in a dialogue between two salesmen from Dubrovnik around 1359, in the 14th-century Vitosha Charter of Bulgarian tsar Ivan Shishman and in a Ragusan merchant's notes of 1376. In these documents, the city is called Sofia,^{[clarification needed]} but, at the same time, the region and the city's inhabitants are still called Sredecheski (Church Slavonic: срѣдечьскои, "of Sredets"), which continued until the 20th century. The Ottomans came to favour the name Sofya (صوفيه). In 1879, there was a dispute about what the name of the new Bulgarian capital should be, when the citizens created a committee of famous people, insisting for the Slavic name. Gradually, a compromise arose, officialisation of Sofia for the nationwide institutions, while legitimating the title Sredets for the administrative and church institutions, before the latter was abandoned through the years.

Sofia City Province has an area of 1344 km², while the surrounding and much bigger Sofia Province is 7,059 km². Sofia's development as a significant settlement owes much to its central position in the Balkans. It is situated in western Bulgaria, at the northern foot of the Vitosha mountain, in the Sofia Valley that is surrounded by the Balkan mountains to the north. The valley has an average altitude of 550 metres (1,800 ft). Sofia is the second highest capital of the European Union (after Madrid) and the third highest capital of Europe (after Andorra la Vella and Madrid). Unlike most European capitals, Sofia does not straddle any large river, but is surrounded by comparatively high mountains on all sides. Three mountain passes lead to the city, which have been key roads since antiquity, Vitosha being the watershed between Black and Aegean Seas.