管弦乐团（英语：Orchestra）是当今世上编制最庞大、最复杂的乐团型态，拥有极强大而广泛之音乐表现力。管弦乐团一般演奏古典音乐或为歌剧伴奏，有时也会替流行音乐伴奏；现代不少管弦乐团也常为电影伴奏，制作电影原声带。

一个管弦乐团大多有70位以上的演奏家组成，有些甚至有上百位演奏家。形式比较小的管弦乐团也被称为室内乐团（Chamber Orchestra），室内乐团的成员数量一般少于30人，在两者中间还有所谓小交响乐团（Sinfonietta Orchestra），主要演奏接近较大于真正室乐和「典型」近代交响乐曲中间的管弦乐作品，如历史上巴洛克乐派或古典乐派交响曲或大协奏曲，其编制是动用30到50人的中型乐团演出。一些大型的管弦乐团也被称为交响乐团（Symphonic Orchestra）或爱乐乐团（Philharmonic Orchestra），两者称谓之间并没有实质上的区别，有时是一个城市中有两队管弦乐团时，可以此来区分彼此，例如伦敦交响乐团和伦敦爱乐乐团。

An orchestra (/ˈɔːrkᵻstrə/ or US /ˈɔːrˌkɛstrə/; Italian: [orˈkɛstra]) is a large instrumental ensemble, often used in classical music, that contains sections of string (violin, viola, cello and double bass), brass, woodwind, and percussion instruments. Other instruments such as the piano and celesta may sometimes be grouped into a fifth section such as a keyboard section or may stand alone, as may the concert harp and, for 20th and 21st century compositions, electric and electronic instruments. The term orchestra derives from the Greek ὀρχήστρα (orchestra), the name for the area in front of an ancient Greek stage reserved for the Greek chorus. The orchestra grew by accretion throughout the 18th and 19th centuries, but changed very little in composition during the course of the 20th century.