Alexey Leonidovich Pajitnov (born April 16, 1955) is a Russian-American computer engineer and video game designer. He is best known for creating, designing, and developing Tetris in 1985 while working at the Dorodnitsyn Computing Centre under the Academy of Sciences of the Soviet Union (now the Russian Academy of Sciences). After Tetris was released internationally in 1987, he released a sequel in 1989, entitled Welltris.

In 1991, he moved to the United States and later became a U.S. citizen. In 1996, Pajitnov founded The Tetris Company alongside Dutch video game designer Henk Rogers. Despite the game's high popularity, Pajitnov did not receive royalties from Tetris prior to this time; the Soviet Union had disintegrated by 1991.

Pajitnov was born to Russian parents who were both writers. His father was an art critic and his mother was a journalist who wrote for both newspapers and a film magazine. It was through his parents that Pajitnov gained exposure to the arts, eventually developing a passion for cinema. He accompanied his mother to many film screenings, including the Moscow Film Festival. Pajitnov was also mathematically inclined, enjoying puzzles and problem solving.

In 1967, when he was 11 years old, Pajitnov's parents divorced. For several years, he lived with his mother in a one-bedroom apartment owned by the state. The two were eventually able to move into a private apartment at 49 Gertsen Street, when Pajitnov was 17. He later went on to study applied mathematics at the Moscow Aviation Institute.

In 1977, Pajitnov worked as a summer intern at the Soviet Academy of Sciences. Once he graduated in 1979, he accepted a job there working on speech recognition at the academy's Dorodnitsyn Computing Centre. When the Computing Centre received new equipment, its researchers would write a small program for it in order to test its computing capabilities. According to Pajitnov, this "became [his] excuse for making games". Computer games were fascinating to him because they offered a way to bridge the gap between logic and emotion, and Pajitnov held interests in both mathematics and puzzles, as well as the psychology of computing.

Searching for inspiration, Pajitnov recalled his childhood memories of playing pentominoes, a game in which the user creates pictures using its shapes. Remembering the difficulty he had in putting the pieces back into their box, Pajitnov felt inspired to create a game based on that concept. Using an Electronika 60 in the Computing Centre, he began working on what would become the first version of Tetris. Building the first prototype in two weeks, Pajitnov spent longer playtesting and adding to the game, completing it on June 6, 1985. This primitive version did not have levels or a scoring system, but Pajitnov knew he had a potentially great game, since he could not stop playing it at work.

The game attracted the interest of coworkers like fellow programmer Dmitri Pevlovsky, who helped Pajitnov connect with Vadim Gerasimov, a 16-year-old intern at the Soviet Academy. Pajitnov wanted to make a color version of Tetris for the IBM Personal Computer, and enlisted the intern to help. Gerasimov created the PC version in less than three weeks, and with contributions from Pevlovsky, spent an additional month adding new features like scorekeeping and sound effects. The game, first available in the Soviet Union, received international releases through Mirrorsoft and Spectrum Holobyte in 1988.

Pajitnov created a sequel to Tetris, entitled Welltris, which has the same principle, but in a three-dimensional environment where the player sees the playing area from above.