Vostok 1 (Russian: Восток, lit. 'East' or 'Orient') was the first spaceflight of the Vostok programme and the first human orbital spaceflight in history. The Vostok 3KA space capsule was launched from Baikonur Cosmodrome on 12 April 1961, with Soviet cosmonaut Yuri Gagarin aboard, making him the first human to reach orbital velocity around the Earth and to complete a full orbit around the Earth.

The orbital spaceflight consisted of a single orbit around Earth which skimmed the upper atmosphere at 169 kilometers (105 miles; 91 nautical miles) at its lowest point. The flight took 108 minutes from launch to landing. Gagarin parachuted to the ground separately from his capsule after ejecting at 7 km (23,000 ft) altitude.

The Space Race between the Soviet Union and the United States, the two Cold War superpowers, began just before the Soviet Union launched the world's first artificial satellite, Sputnik 1, in 1957. Both countries wanted to develop spaceflight technology quickly, particularly by launching the first successful human spaceflight. The Soviet Union secretly pursued the Vostok programme in competition with the United States' Project Mercury. Vostok launched several precursor uncrewed missions between May 1960 and March 1961, to test and develop the Vostok rocket family and space capsule. These missions had varied degrees of success, but the final two—Korabl-Sputnik 4 and Korabl-Sputnik 5—were complete successes, allowing the first crewed flight.

The Vostok 1 capsule was designed to carry a single cosmonaut. Yuri Gagarin was chosen as the prime pilot of Vostok 1, with Gherman Titov and Grigori Nelyubov as backups. These assignments were formally made on April 8, four days before the mission, but Gagarin had been a favourite among the cosmonaut candidates for at least several months.

The final decision of who would fly the mission relied heavily on the opinion of the head of cosmonaut training, Nikolai Kamanin. In an April 5 diary entry, Kamanin wrote that he was still undecided between Gagarin and Titov. "The only thing that keeps me from picking [Titov] is the need to have the stronger person for the one day flight." Kamanin was referring to the second mission, Vostok 2, compared to the relatively short single-orbit mission of Vostok 1. When Gagarin and Titov were informed of the decision during a meeting on April 9, Gagarin was very happy, and Titov was disappointed. On 10 April, this meeting was reenacted in front of television cameras, so there would be official footage of the event. This included an acceptance speech by Gagarin. As an indication of the level of secrecy involved, one of the other cosmonaut candidates, Alexei Leonov, later recalled that he did not know who was chosen for the mission until after the spaceflight had begun.

Unlike later Vostok missions, there were no dedicated tracking ships available to receive signals from the spacecraft. Instead they relied on the network of ground stations, also called Command Points, to communicate with the spacecraft; all of these Command Points were located within the Soviet Union.

Because of weight constraints, there was no backup retrorocket engine. The spacecraft carried 13 days of provisions to allow for survival and natural orbital decay in the event the retrorockets failed. The provisions included food for Gagarin. As focus was made on food that would not form crumbs, Gagarin was provided with liver meat puree and chocolate sauce, packed in metal toothpaste-style tubes.

The letters "СССР" were hand-painted onto Gagarin's helmet by engineer Gherman Lebedev during transfer to the launch site. As it had been less than a year since U-2 pilot Francis Gary Powers was shot down, Lebedev reasoned that without some country identification, there was a small chance the cosmonaut might be mistaken for a spy on landing.