Anderssen's Opening is a chess opening defined by the opening move: 1. a3.
The opening is named after unofficial World Chess Champion Adolf Anderssen, who played it three times in his 1858 match against Paul Morphy. While Anderssen was defeated decisively in the match, the games he opened with this novelty scored 1½/3 (one win, one loss, one draw).
This opening move does little for development or control of the center. In some cases, White can transpose the game to an opening where 1.a3 might have been useful, but using a tempo on such a move already on move one seems premature. In fact, this opening is based on the idea that White is playing with the black pieces, but he has the move 1.a3 already played. If a game starts 1.a3 e5 2.e4 Nf6 3.Nc3, Black cannot proceed in Ruy Lopez-fashion, and if Black plays 3...Bc5, then 4.Nf3 puts Black into the Two Knights' Defence and White's a3 precludes many possibilities.
Anderssen's Opening is not a very constructive move for White, more a waiting move. Some players may enjoy the psychological value of such a move, however, or believe it will help them against an opponent with a superior knowledge of opening theory. As Anderssen's Opening is not commonly played, it is considered an irregular opening.