Lego (/ˈlɛɡoʊ/ LEG-oh, Danish: [ˈle̝ːko]; stylised as LEGO) is a line of plastic construction toys manufactured by the Lego Group, a privately held company based in Billund, Denmark. Lego consists of variously coloured interlocking plastic bricks made of acrylonitrile butadiene styrene (ABS) that accompany an array of gears, figurines called minifigures, and various other parts. Its pieces can be assembled and connected in many ways to construct objects, including vehicles, buildings, and working robots. Assembled Lego models can be taken apart, and their pieces can be reused to create new constructions.

The Lego Group began manufacturing the interlocking toy bricks in 1949. Moulding is done in Denmark, Hungary, Mexico, and China. Brick decorations and packaging are done at plants in the former three countries and in the Czech Republic. Annual production of the bricks averages approximately 36 billion, or about 1140 elements per second. One of Europe's biggest companies, Lego is the largest toy manufacturer in the world by sales. As of July 2015, 600 billion Lego parts had been produced. Lego maintains a large fan community based around building competitions and custom creations, and a range of films, games, and eight Legoland amusement parks have been developed under the brand.

One of the largest commercially produced Lego sets was a minifigure-scaled edition of the Star Wars Millennium Falcon. Designed by Jens Kronvold Fredericksen, it was released in 2007 and contained 5,195 pieces. It was surpassed by a 5,922-piece Taj Mahal. A redesigned Millennium Falcon retook the top spot in 2017 with 7,541 pieces. Since then, the Millennium Falcon has been superseded by the Lego Art World Map at 11,695 pieces, the Lego Titanic at 9,090 pieces, and the Lego Architect Colosseum at 9,036 pieces.

The Lego Group began in the workshop of Ole Kirk Christiansen (1891–1958), a carpenter from Billund, Denmark, who began making wooden toys in 1932. In 1934, his company came to be called "Lego", derived from the Danish phrase leg godt [lɑjˀ ˈkʌt], which means "play well". In 1947, Lego expanded to begin producing plastic toys. In 1949 the business began producing, among other new products, an early version of the now familiar interlocking bricks, calling them "Automatic Binding Bricks". These bricks were based on the Kiddicraft Self-Locking Bricks, invented by Hilary Page in 1939 and patented in the United Kingdom in 1940 before being displayed at the 1947 Earl's Court Toy Fair. Lego had received a sample of the Kiddicraft bricks from the supplier of an injection-molding machine that it purchased. The bricks, originally manufactured from cellulose acetate, were a development of the traditional stackable wooden blocks of the time.

In May 2011, Space Shuttle Endeavour mission STS-134 brought 13 Lego kits to the International Space Station, where astronauts built models to see how they would react in microgravity, as a part of the Lego Bricks in Space program. In May 2013, the largest model ever created, made of over 5 million bricks, was displayed in New York City; a one-to-one scale model of a Star Wars X-wing fighter. Other record breakers include a 34-metre (112 ft) tower and a 4 km (2.5 mi) railway. In February 2015, marketing consulting company Brand Finance ranked Lego as the "world's most powerful brand", overtaking Ferrari.

While Lego has generally been considered a children's toy, there have also been adult fans of the toys. In 2020, Lego introduced sets aged at 18+, generally some of their more expensive and difficult-to-assemble sets based on real world or fictional objects, such as the Concorde or Rivendell. The timing of these sets favorably aligned with the COVID-19 pandemic, with many adults purchasing these sets to work on during various lockdown periods. Popularity within adults was further pushed by the release of The Lego Movie and the reality series Lego Masters. By 2024, nearly 15% of the sets released in the U.S. were aimed for adult builders.