Laika (/ˈlaɪkə/ LY-kə; Russian: Лáйка, IPA: [ˈlajkə]; c. 1954 – 3 November 1957) was a Soviet space dog who was one of the first animals in space and the first to orbit the Earth. A stray mongrel from the streets of Moscow, she flew aboard the Sputnik 2 spacecraft, launched into low orbit on 3 November 1957. As the technology to re-enter the atmosphere had not yet been developed, Laika's survival was never expected. She died of hyperthermia hours into the flight, on the craft's fourth orbit.

Little was known about the effects of spaceflight on living creatures at the time of Laika's mission, and animal flights were viewed by engineers as a necessary precursor to human missions. The experiment, which monitored Laika's vital signs, aimed to prove that a living organism could survive being launched into orbit and continue to function under conditions of weakened gravity and increased radiation, providing scientists with some of the first data on the biological effects of spaceflight.

Laika's death was possibly caused by a failure of the central R‑7 sustainer to separate from the payload. The true cause and time of her death were not made public until 2002; instead, it was widely reported that she died when her oxygen ran out on day six or, as the Soviet government initially claimed, she was euthanised prior to oxygen depletion. In 2008, a small monument to Laika depicting her standing atop a rocket was unveiled near the military research facility in Moscow that prepared her flight. She also appears on the Monument to the Conquerors of Space in Moscow.

After the success of Sputnik 1 in October 1957, Nikita Khrushchev, leader of the Soviet Union, wanted a spacecraft launched on 7 November 1957 to celebrate the 40th anniversary of the October Revolution. Khrushchev specifically wanted to deliver a "space spectacular", a mission that would repeat the triumph of Sputnik 1, stunning the world with Soviet prowess.

While construction had already started on Sputnik 3, a more sophisticated satellite, it would not be ready until December. To meet the November deadline, a new simple satellite would need to be built. Sergei Korolev proposed that a dog be placed in the satellite, an idea which was quickly adopted by planners. Soviet rocket engineers had long intended a canine orbit before attempting human spaceflight; since 1951, they had lofted 12 dogs into sub-orbital space on ballistic flights, working gradually toward an orbital mission set for some time in 1958. To satisfy Khrushchev's demands, they expedited the orbital canine flight for the November launch.

According to Russian sources, the official decision to launch Sputnik 2 was made on 10 or 12 October, leaving less than four weeks to design and build the spacecraft. Sputnik 2, therefore, was something of a rushed job, with most elements of the spacecraft being constructed from rough sketches. Aside from the primary mission of sending a living passenger into space, Sputnik 2 also contained instrumentation for measuring solar irradiance and cosmic rays.

The craft was equipped with a life-support system consisting of an oxygen generator and devices to avoid oxygen poisoning and to absorb carbon dioxide. A fan, designed to activate whenever the cabin temperature exceeded 15 °C (59 °F), was added to keep the dog cool. Enough food (in a gelatinous form) was provided for a seven-day flight, and the dog was fitted with a bag to collect waste. A harness was designed to be fitted to the dog, and there were chains to restrict her movements to standing, sitting, or lying down; there was no room to turn around in the cabin. An electrocardiogram monitored heart rate and further instrumentation tracked respiration rate, maximum arterial pressure, and the dog's movements.

Laika was found as a stray dog wandering the streets of Moscow. Soviet scientists chose to use Moscow strays since they assumed that such animals had already learned to endure conditions of extreme cold and hunger. She was a five-kilogram (11 lb) mongrel female, approximately three years old. Another account reported that she weighed about six kilograms (13 lb). Soviet personnel gave her several names and nicknames, among them Kudryavka (Russian for Little Curly), Zhuchka (Little Bug), and Limonchik (Little Lemon). Laika, the Russian name for several breeds of dogs similar to the husky, was the name popularised around the world. Its literal translation would be "Barker", from the Russian verb "layat" (лаять), "to bark". According to some accounts, the technicians actually renamed her from Kudryavka to Laika owing to her loud barking. The American press dubbed her Muttnik (mutt + suffix -nik) as a pun on Sputnik, or referred to her as Curly. Her true pedigree is unknown, although it is generally accepted that she was part husky or other Nordic breed, and possibly part terrier. NASA refers to Laika as a "part-Samoyed terrier". A Russian magazine described her temperament as phlegmatic, saying that she did not quarrel with other dogs.