Molenbeek-Saint-Jean (French, pronounced [molœnbek sɛ̃ ʒɑ̃] ^ⓘ) or Sint-Jans-Molenbeek (Dutch, pronounced [sɪɲˈtɕɑns ˈmoːlə(m)ˌbeːk] ^ⓘ), often simply called Molenbeek, is one of the 19 municipalities of the Brussels-Capital Region, Belgium. Located in the western part of the region, it is bordered by the City of Brussels, from which it is separated by the Brussels–Charleroi Canal, as well as by the municipalities of Anderlecht, Berchem-Sainte-Agathe, Dilbeek, Jette, and Koekelberg. The Molenbeek brook, from which it takes its name, flows through the municipality. In common with all of Brussels' municipalities, it is legally bilingual (French–Dutch).

From its origins in the Middle Ages until the 18th century, Molenbeek was a rural village on the edge of Brussels, but around the turn of the 19th century, it experienced major growth brought on by a boom in commerce and manufacturing during the Industrial Revolution. Its prosperity declined after the Second World War, owing to deindustrialisation, leading to extensive investment and regeneration. Experiencing a strong movement of immigration, mainly Moroccan, from the 1950s and 1960s, Molenbeek became increasingly multicultural with a minority Muslim population. In the 2010s, it gained international attention as the base for Islamist terrorists who carried out attacks in both Paris and Brussels.

Molenbeek is a mostly residential municipality consisting of several historically and architecturally distinct districts. As of 1 January 2024^[update], the municipality had a population of 98,365 inhabitants. The total area is 6.02 km² (2.32 sq mi), which gives a population density of 16,352/km² (42,350/sq mi), twice the average of Brussels. Its upper area is greener and less densely populated.

The name Molenbeek derives from two Dutch words: molen, meaning "mill", and beek, meaning "brook"; and could be literally translated as "Millbrook" in English. It is a very common name for brooks in the Netherlands and Belgium, such as the Molenbeek (Erpe-Mere Bovenschelde), as well as the Molenbeek-Ter Erpenbeek, both in the Denderstreek, Belgium.

Although first applied to the brook that flowed through the village, the name Molenbeek (originally spelled Molembecca) eventually came to be used to designate the village itself, around the year 985. The suffix Saint-Jean in French or Sint-Jans in Dutch, meaning "Saint John", refers to the parish's patron saint, Saint John the Baptist, though it is seldom used in everyday speech, today's inhabitants—whether French or Dutch speaking—usually shortening the name to simply Molenbeek.

In French, Molenbeek-Saint-Jean is pronounced [molənbeːk sɛ̃ ʒɑ̃], and in Dutch, Sint-Jans-Molenbeek is pronounced [sɪɲˈtɕɑns ˈmoːlə(m)ˌbeːk] ^ⓘ (in both languages, the "-beek" is pronounced like "bake" in English). Inhabitants of Molenbeek are known in French as Molenbeekois (pronounced [molənbeːkwa]) and in Dutch as Molenbeekenaars (pronounced [ˈmoːlə(m)ˌbeːkənaːrs]). In France, the pronunciations [molənbɛk sɛ̃ ʒɑ̃] ("-beek" like "beck" in English) and [molənbɛkwa] (for molenbeekois) are often heard, but are rather rare in Belgium. The dialectal forms Muilebeik and Meulebeik are still used by older adults of Belgian ancestry, whilst the abbreviations Molen and Molem are common among younger speakers.

As early as the 9th century, Molenbeek was the site of a church dedicated to Saint John the Baptist. The parish boundaries of St. John's Church were much greater than today, reaching as far as the river Senne, and from the end of the 12th century, included a chapel dedicated to Saint Catherine. This chapel was split off from the rest of the parish following the construction of Brussels' second city walls and gradually became the current Church of St. Catherine in the Quays or Sainte-Catherine/Sint-Katelijne Quarter of Brussels. The first documented mention of Molenbeek was made on 9 April 1174 in a papal bull by Pope Alexander III listing the property of the chapter of the Collegiate Church of St. Michael and St. Gudula (now Brussels' cathedral), which included St. John's Church, as well as other property. The Brussels Beguinage, founded before 1247 outside the city walls, also depended on Molenbeek.

In the early Middle Ages, Molenbeek was known for its miraculous spring of Saint Gertrude of Nivelles, the mythical founder of Nivelles Abbey, which attracted thousands of pilgrims. According to legend, she visited Molenbeek and offered the land on which the village's first church was built, and allegedly caused this sacred spring to spur out by ramming her abbess's crosier into the ground near the church. Later, the tradition of a special pilgrimage for patients with epilepsy developed around St. John's Church. On St. John's Day (24 June), a dancing procession took place, in which epileptics could be freed from their illness for a year if they crossed a bridge over the Molenbeek brook towards the church without their feet touching the ground. A painting by Pieter Brueghel the Younger, dating from 1592, illustrates this procession.