The London Sinfonietta is an English contemporary chamber orchestra founded in 1968 and based in London.

The ensemble has headquarters at Kings Place and is Resident Orchestra at the Southbank Centre. Since its inaugural concert in 1968—giving the world premiere of Sir John Tavener’s The Whale—the London Sinfonietta's commitment to making new music has seen it commission over 300 works, and premiere many hundreds more.

The core of the London Sinfonietta is its 18 Principal Players. In September 2013 the ensemble launched its Emerging Artists Programme.

The London Sinfonietta's recordings comprise a catalogue of 20th-century classics, on numerous labels as well as the ensemble's own London Sinfonietta Label.