Petrosian was a conservative, cautious, and highly defensive chess player who was strongly influenced by Aron Nimzowitsch’s idea of prophylaxis. He made more effort to prevent his opponent’s offensive capabilities than he did to make use of his own, and very rarely went on the offensive unless he felt his position was completely secure. He usually won by playing consistently until an overly aggressive opponent made a mistake, securing victory by capitalizing upon this mistake without revealing any weaknesses of his own. This style of play often led to draws, especially against other players who preferred to counterattack. Nonetheless, his patience and mastery of defense made him extremely difficult to defeat. He was undefeated at the 1952 and 1955 Interzonals, and in 1962 he did not lose a single tournament game. Petrosian’s consistent ability to avoid defeat earned him the nickname “Iron Tigran.” He was considered to be the hardest player to beat in the history of chess by the authors of a 2004 book, and future World Champion Vladimir Kramnik called him “the first defender with a capital D”.

Petrosian preferred to play closed openings that did not commit his pieces to any particular plan. As black, Petrosian enjoyed playing the Sicilian Defence, Najdorf Variation, and the French Defence. As white, he often played the English Opening. Petrosian would often move the same piece multiple times in a few moves, confusing his opponents in the opening and threatening draws by threefold repetition in the endgame. In a game against Mark Taimanov during the 1955 USSR Chess Championship, Petrosian moved the same rook six times in a 24-move game, with four of those moves occurring on consecutive turns. He had a strong affinity for knights rather than bishops, a characteristic that is often attributed to the influence of Aron Nimzowitsch.

A number of illustrative metaphors have been used to describe Petrosian’s style of play. Harold C. Schonberg said that “playing him was like trying to put handcuffs on an eel. There was nothing to grip.” He has been described as a centipede lurking in the dark, a tiger looking for the opportunity to pounce, a python who slowly squeezes his victims to death, and as a crocodile who waits for hours to make a decisive strike. Boris Spassky, who succeeded Petrosian as World Chess Champion, described his style of play thus: “Petrosian reminds me of a hedgehog. Just when you think you have caught him, he puts out his quills.”

Petrosian’s style of play, although highly successful for avoiding defeats, was criticized as being dull. Chess enthusiasts saw his “ultraconservative” style as an unwelcome contrast to the popular image of Soviet chess as “daring” and “indomitable”. His 1971 Candidates Tournament match with Viktor Korchnoi featured so many monotonous draws that the Russian press began to complain. However, Svetozar Gligorić described Petrosian as being “very impressive in his incomparable ability to foresee danger on the board and to avoid any risk of defeat.” Petrosian responded to his criticisms by saying: “They say my games should be more ‘interesting’. I could be more ‘interesting’—and also lose.”

Another consequence of Petrosian’s style of play was that he did not score many victories, which in turn meant he seldom won tournaments even though he often finished second or third. However, his style was extremely effective in matches.

Petrosian could also occasionally play in an attacking, sacrificial style. In his 1966 match with Spassky, he won Game 7 and Game 10 this way. Boris Spassky subsequently stated: “It is to Petrosian’s advantage that his opponents never know when he is suddenly going to play like Mikhail Tal.” (Tal was known as the most aggressive attacker of his era.)