Petrov's Defence or the Petrov Defence (also called Petroff Defence, Petrov's Game, Russian Defence, or Russian Game [Russian: Русская партия]) is a chess opening characterised by the following moves:

Although this symmetrical response has a long history, it was first popularised by Alexander Petrov, a Russian chess player in the mid-19th century. In recognition of the early investigations by the Russian masters Petrov and Carl Jaenisch, this opening is called the Russian Game in some countries.

The Petrov has a drawish reputation; however, it offers attacking opportunities for both sides, and a few lines are quite sharp. Often a trade occurs and Black, after gaining a tempo, has a well-placed knight. Pillsbury's game in 1895 against Emanuel Lasker testifies to this. The Black counterattack in the centre also avoids the Ruy Lopez, Giuoco Piano (and other lines of the Italian Game), and the Scotch Game. The Petrov has been adopted by many of the world's leading players, including world champions Vasily Smyslov, Tigran Petrosian, Anatoly Karpov, and Vladimir Kramnik, along with grandmaster Fabiano Caruana and others.

The Encyclopaedia of Chess Openings codes for Petrov's Defence are C43 (for 3.d4) and C42 (for all other lines).

White's two main choices are 3.Nxe5 and 3.d4. 3.Nxe5 is more popular but they often lead to similar positions and "there is no clear reason to prefer one move over the other".

3.Nc3 may transpose to the Four Knights Game. 3.Bc4 may lead to the Boden–Kieseritzky Gambit or transpose to the Two Knights Defence or Bishop's Opening. Occasionally seen is the quiet 3.d3.

Examined in this article are the following common variations:

After 3.Nxe5, the Classical Variation, the standard reply for Black is 3...d6. After 4.Nf3 Nxe4, White's main options are 5.d4, intending to undermine the knight on e4, and 5.Nc3, aiming for rapid development with queen's side castling and a king's side attack. Other moves offer little hope for an advantage.