Gemini is one of the constellations of the zodiac and is located in the northern celestial hemisphere. It was one of the 48 constellations described by the 2nd century AD astronomer Ptolemy, and it remains one of the 88 modern constellations today. Its name is Latin for twins, and it is associated with the twins Castor and Pollux in Greek mythology. Its old astronomical symbol is (♊︎).

Gemini lies between Taurus to the west and Cancer to the east, with Auriga and Lynx to the north, Monoceros and Canis Minor to the south, and Orion to the south-west.

In classical antiquity, Cancer was the location of the Sun on the northern solstice (June 21). During the first century AD, axial precession shifted it into Gemini. In 1990, the location of the Sun at the northern solstice moved from Gemini into Taurus, where it will remain until the 27th century AD and then move into Aries. The Sun will move through Gemini from June 21 to July 20 through 2062.

Gemini is prominent in the winter skies of the northern Hemisphere and is visible the entire night in December–January. The easiest way to locate the constellation is to find its two brightest stars Castor and Pollux eastward from the familiar V-shaped asterism (the open cluster Hyades) of Taurus and the three stars of Orion's Belt (Alnitak, Alnilam, and Mintaka). Another way is to mentally draw a line from the Pleiades star cluster located in Taurus and the brightest star in Leo, Regulus. In doing so, an imaginary line that is relatively close to the ecliptic is drawn, a line which intersects Gemini roughly at the midpoint of the constellation, just below Castor and Pollux.

When the Moon moves through Gemini, its motion can easily be observed in a single night as it appears first west of Castor and Pollux, then aligns, and finally appears east of them.

Bayer gave the most prominent stars Bayer designations Alpha through Omega and the Latin letters A through g. Later designated two stars as b. Psi Geminorum was dropped after moving it into Cancer in 1930; however, Flamsteed used Psi³ Cancri on this star, leading into confusion.

The constellation contains 85 stars of naked eye visibility. The brightest star in Gemini is Pollux, and the second-brightest is Castor. Castor's Bayer designation as "Alpha" arose because Johann Bayer did not carefully distinguish which of the two was the brighter when he assigned his eponymous designations in 1603. Although the characters of myth are twins, the actual stars are physically very different from each other.

α Gem (Castor) is a sextuple star system 52 light-years from Earth, which appears as a magnitude 1.6 blue-white star to the unaided eye. Two spectroscopic binaries are visible at magnitudes 1.9 and 3.0 with a period of 470 years. A wide-set red dwarf star is also a part of the system; this star is an Algol-type eclipsing binary star with a period of 19.5 hours; its minimum magnitude is 9.8 and its maximum magnitude is 9.3.