Baalbek (/ˈbɑːlbɛk, ˈbeɪəlbɛk/; Arabic: بَعْلَبَكّ, romanized: Baʿlabakk; Syriac: ܒܥܠܒܟ) is a city located east of the Litani River in Lebanon's Beqaa Valley, about 67 km (42 mi) northeast of Beirut. It is the capital of Baalbek-Hermel Governorate. In 1998, the city had a population of 82,608. Most of the population consists of Shia Muslims, followed by Sunni Muslims and Christians; in 2017, there was also a large presence of Syrian refugees.

Baalbek's history dates back at least 11,000 years. After Alexander the Great conquered the city in 334 BCE, he renamed it Heliopolis (Ἡλιούπολις, Greek for "Sun City"). The city flourished under Roman rule. However, it underwent transformations during the Christianization period and the subsequent rise of Islam following the Arab conquest in the 7th century. In later periods, the city was sacked by the Mongols and faced a series of earthquakes, resulting in a decline in importance during the Ottoman and modern periods.

In the modern era, Baalbek is a tourist destination. It is known for the ruins of the Roman temple complex, which includes the Temple of Bacchus and the Temple of Jupiter, and was inscribed in 1984 as a UNESCO World Heritage site. Other tourist attractions are the Great Umayyad Mosque, the Baalbek International Festival, the mausoleum of Sit Khawla, and a Roman quarry site named Hajar al-Hibla. Baalbek's tourism sector has encountered challenges due to conflicts in Lebanon, particularly the 1975–1990 civil war, the ongoing Syrian civil war since 2011, and the Israel–Hezbollah conflict (2023–present).

Baalbek is considered to be part of Hezbollah's heartland and is known to be their political stronghold. During the Israeli occupation of Southern Lebanon (1982–2000), the group managed to overpower the Lebanese army in Baalbek and gain control of the city. The settlement was subsequently used as a base to recruit and train men for attacks against Israeli forces. Hezbollah continues to hold significant political influence and popular support in Baalbek. In the 2022 Lebanese general election the Amal-Hezbollah list won 9 out of 10 seats in the Baalbek-Hermel Governorate.

Israel has conducted numerous airstrikes and raids against military and civilian targets in the Baalbek area in the past decades. For instance, in 2006 during the Operation Sharp and Smooth, Israeli commandos raided a hospital and bombed multiple houses, killing two Hezbollah fighters and at least eleven civilians. In 2024, during the Israel–Hezbollah conflict, Israel sent forced displacement calls for the entire city. Shortly after, Israeli airstrikes killed 19 people, including 8 women.

A few kilometres from the swamp from which the Litani (the classical Leontes) and the Asi (the upper Orontes) flow, Baalbek may be the same as the manbaa al-nahrayn ("Source of the Two Rivers"), the abode of El in the Ugaritic Baal Cycle discovered in the 1920s and a separate serpent incantation.

Baalbek was called Heliopolis during the Roman Empire, a latinisation of the Greek Hēlioúpolis (Ἡλιούπολις) used during the Hellenistic period, meaning "Sun City" in reference to the solar cult there. The name is attested under the Seleucids and Ptolemies. However, Ammianus Marcellinus notes that earlier Assyrian names of Levantine towns continued to be used alongside the official Greek ones imposed by the Diadochi, who were successors of Alexander the Great. In Greek religion, Helios was both the sun in the sky and its personification as a god. The local Semitic god Baʿal Haddu was more often equated with Zeus or Jupiter or simply called the "Great God of Heliopolis", but the name may refer to the Egyptians' association of Baʿal with their great god Ra. It was sometimes described as Heliopolis in Syria or Coelesyria (Latin: Heliopolis Syriaca or Syriae) to distinguish it from its namesake in Egypt. In Catholicism, its titular see is distinguished as Heliopolis in Phoenicia, from its former Roman province Phoenice. The importance of the solar cult is also attested in the name Biḳāʿ al-ʿAzīz borne by the plateau surrounding Baalbek, as it references an earlier solar deity named Aziz. In Greek and Roman antiquity, it was known as Heliopolis. Some of the best-preserved Roman ruins in Lebanon are located here, including one of the largest temples of the Roman empire. The gods worshipped there (Jupiter, Venus, and Bacchus) were equivalents of the Canaanite deities Hadad, Atargatis. Local influences are seen in the planning and layout of the temples, which differ from classic Roman design.

The name BʿLBK appears in the Mishnah, a second-century rabbinic text, as a kind of garlic, shum ba'albeki (שום בעלבכי). It appears in two early 5th-century Syriac manuscripts, a c. 411 translation of Eusebius's Theophania and a c. 435 life of Rabbula, bishop of Edessa. It was pronounced as Baʿlabakk (Arabic: بَعْلَبَكّ) in Classical Arabic. In Modern Standard Arabic, its vowels are marked as Baʿlabak (بَعْلَبَك) or Baʿlabekk. It is Bʿalbik (بْعَلْبِك, is [ˈbʕalbik]) in Lebanese Arabic.