The Réti Opening (also called the Zukertort Opening and the King's Knight Opening) is a chess opening characterized by the opening move 1.Nf3. It is named after Richard Réti, a Czechoslovakian chess player who used it to defeat the world champion Jose Raul Capablanca in 1924. In the Encyclopaedia of Chess Openings (ECO) Réti Opening has the code A04.

According to ChessBase, out of the twenty possible opening moves, 1.Nf3 ranks third in popularity. It develops the knight to a good square and prepares for a quick castling, White does not commit to a particular central pawn structure, instead waiting to see what Black will do. The slight drawback to the move is that it blocks the f-pawn. This is not a problem if White does not intend to move it in the near future, but it rules out the possibility of playing systems with f3 and Nge2, which is a fairly popular setup against the King's Indian.

Usually 1.Nf3 will transpose into an opening with 1.d4, such as the King's Indian or the Queen's Gambit. If White follows up with an early c4 a transposition to the English Opening may be reached. Even the Sicilian Defense may be reached if the game opens 1.Nf3 c5 2.e4.

When the game does not transpose to some other opening, the main lines to Réti Opening are

• 1..Nf6 (ECO code A05)
• 1..d5 (A06)
  • 2.g3 (King's Indian Attack, A07)
    • 2..c5 3.Bg2 (King's Indian Attack, A08)
  • 2.c4 (A09)