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Anderssen's Opening is a chess opening defined by the opening move 1.a3 in algebraic notation. It is named after Adolf Anderssen who played it three times in his 1858 match against Paul Morphy.

This opening move is rare because it does little or nothing for development or control of the center. In some cases White will be able to transpose the game to an opening where 1.a3 will have been useful, but using a tempo on such a move already on move one is premature. As Anderssen's Opening is rarely played, it is considered an irregular opening, so it is classified under the A00 code in the Encyclopaedia of Chess Openings.

Still 1.a3 is not the worst opening move. Although it is not a very constructive move for White it is not a destructive one, unlike 1.f3 (Barnes Opening). Some players may enjoy the psychological effect of such a move, or believe it will help them against an opponent with a superior knowledge of opening theory.

Among the more common Black responses to Anderssen's Opening are 1...d5 which makes a straightforward claim of the center, and 1...g6 which prepares to fianchetto a bishop to point down on White's slightly weakened queenside. The response 1...e5 is also possible, but White can then play 2.c4, leading to a kind of Sicilian Defense with colors reversed, where a pawn on a3 can be useful.