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This article uses algebraic notation to describe chess moves.

Larsen's Opening, also called the Nimzo-Larsen Attack or Queen's Fianchetto Opening, is a chess opening starting with

1.b3

It is named after the Danish Grandmaster Bent Larsen. Larsen was inspired by the example of the great Latvian-Danish player and theoretician Aron Nimzowitsch (1886-1935), who often played 1.Nf3 followed by 2.b3, which is sometimes called the Nimzowitsch-Larsen Attack. It is classified under the A01 code in the Encyclopaedia of Chess Openings.

The opening move 1.b3 prepares to fianchetto the queen's bishop where it will fight for the central squares and point towards Black's kingside. Still, 1.b3 is less popular than 1.g3 (Benko's Opening), which prepares a quick kingside castling. According to ChessBase, 1.b3 ranks sixth in popularity out of the possible twenty first moves while the fifth-ranking 1.g3 is about three times as popular.

Although Bent Larsen was initially very successful with this opening, it suffered a setback in the 1970 USSR vs. Rest of the World match in Belgrade. There, Larsen played this opening against reigning World Champion Boris Spassky, and lost in just 17 moves, a remarkably quick loss, especially when playing White. Larsen was also decisively defeated when playing this opening against Balinas at Manila 1975.

Black has several options to meet 1.b3. The most common are

• 1...e5 is the most common response, making a grab for the centre and limiting the scope of the White Bishop.
• 1...d5 is the second most common, also making a grab for the centre and preserving the option to fianchetto the King's Bishop to oppose the White one.
• 1...Nf6 developing a piece and not committing to a particular pawn formation just yet.
• 1...c5, retaining the options of ...d5, or ...d6 followed by ...e5, e.g., 2.Bb2 d6 3.Nf3 e5.

• Bent Larsen v. Boris Spassky, Beograd 1970

Wikibooks' Opening theory in chess has more about this subject:

1. b3