Sputnik 1 (/ˈspʌtnɪk, ˈspʊtnɪk/, Russian: Спутник-1, Satellite 1), sometimes referred to as simply Sputnik, was the first artificial Earth satellite. It was launched into an elliptical low Earth orbit by the Soviet Union on 4 October 1957 as part of the Soviet space program. It sent a radio signal back to Earth for three weeks before its three silver-zinc batteries became depleted. Aerodynamic drag caused it to fall back into the atmosphere on 4 January 1958.

It was a polished metal sphere 58 cm (23 in) in diameter with four external radio antennas to broadcast radio pulses. Its radio signal was easily detectable by amateur radio operators, and the 65° orbital inclination made its flight path cover virtually the entire inhabited Earth.

The satellite's success was unanticipated by the United States. This precipitated the American Sputnik crisis and triggered the Space Race. The launch was the beginning of a new era of political, military, technological, and scientific developments. The word sputnik is Russian for satellite when interpreted in an astronomical context; its other meanings are spouse or travelling companion.

Tracking and studying Sputnik 1 from Earth provided scientists with valuable information. The density of the upper atmosphere could be deduced from its drag on the orbit, and the propagation of its radio signals gave data about the ionosphere.

Sputnik 1 was launched during the International Geophysical Year from Site No.1/5, at the 5th Tyuratam range, in Kazakh SSR (now known as the Baikonur Cosmodrome). The satellite travelled at a peak speed of about 8 km/s (18,000 mph), taking 96.20 minutes to complete each orbit. It was transmitted on 20.005 and 40.002 MHz, which were monitored by radio operators throughout the world. The signals continued for 22 days until the transmitter batteries depleted on 26 October 1957. On 4 January 1958, after three months in orbit, Sputnik 1 burned up while reentering Earth's atmosphere, having completed 1,440 orbits of the Earth, and travelling a distance of approximately 70,000,000 km (43,000,000 mi).

Спутник-1, romanized as Sputnik-Odin (pronounced [ˈsputnʲɪk.ɐˈdʲin]), means 'Satellite-One'. The Russian word for satellite, sputnik, was coined in the 18th century by combining the prefix s- ('fellow') and putnik ('traveler'), thereby meaning 'fellow-traveler', a meaning corresponding to the Latin root satelles ('guard, attendant or companion'), which is the origin of English satellite. (In the West, dating from the 1930s, the term fellow traveler was already used as a pejorative to describe people who were philosophically sympathetic to communism.)

In English, 'Sputnik' is widely recognized as a proper name; however, this is not the case in Russian. In the Russian language, sputnik is the general term for the artificial satellites of any country and the natural satellites of any planet. The incorrect attribution of 'Sputnik' as a proper name can be traced back to an article released by The New York Times on 6 October 1957, titled "Soviet 'Sputnik' Means A Traveler's Traveler". In the referenced article, the term 'Sputnik' was portrayed as bearing a poetic connotation arising from its linguistic origins. This connotation incorrectly indicated that it was bestowed with the specific proper name 'Fellow-Traveler-One', rather than being designated by the general term 'Satellite-One'. In Russian-language references, Sputnik 1 is recognised by the technical name of 'Satellite-One'.

On 17 December 1954, chief Soviet rocket scientist Sergei Korolev proposed a developmental plan for an artificial satellite to the Minister of the Defense Industry, Dimitri Ustinov. Korolev forwarded a report by Mikhail Tikhonravov, with an overview of similar projects abroad. Tikhonravov had emphasised that the launch of an orbital satellite was an inevitable stage in the development of rocket technology.