Circinus is a small, faint constellation in the southern sky, first defined in 1756 by the French astronomer Nicolas-Louis de Lacaille. Its name is Latin for compass, referring to the drafting tool used for drawing circles (it should not be confused with Pyxis, a constellation that represents a mariner's compass which points north). Its brightest star is Alpha Circini, with an apparent magnitude of 3.19. Slightly variable, it is the brightest rapidly oscillating Ap star in the night sky. AX Circini is a Cepheid variable visible with the unaided eye, and BX Circini is a faint star thought to have been formed from the merger of two white dwarfs. Two sun-like stars have planetary systems: HD 134060 has two small planets, and HD 129445 has a Jupiter-like planet. Supernova SN 185 appeared in Circinus in 185 AD and was recorded by Chinese observers. Two novae have been observed more recently, in the 20th century.

The Milky Way runs through the constellation, featuring prominent objects such as the open cluster NGC 5823 and the planetary nebula NGC 5315. Circinus hosts a notable spiral galaxy, the Circinus Galaxy, discovered in 1977; it is the closest Seyfert galaxy to the Milky Way. The Alpha Circinids (ACI), a meteor shower also discovered in 1977, radiate from this constellation.

In 1756, French astronomer Nicolas-Louis de Lacaille introduced the constellation of Circinus with the French name le Compas, representing a pair of dividing compasses, on a chart of the southern sky. On that chart, Lacaille portrayed the constellations of Norma, Circinus, and Triangulum Australe, respectively, as a set square and ruler, a compass, and a surveyor's level in a set of draughtsman's instruments. Circinus was given its current name in 1763, when Lacaille published an updated sky map with Latin names for the constellations he introduced.

Bordered by Centaurus, Musca, Apus, Triangulum Australe, Norma and Lupus, Circinus lies adjacent to the Alpha and Beta Centauri stars. As it is at declination −50° to −70°, the whole constellation is only visible south of latitude 30° N. The official constellation boundaries, as set by Belgian astronomer Eugène Delporte in 1930, are defined by a polygon of 14 segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 13^h 38.4^m and 15^h 30.2^m , and the declination coordinates are between −55.43° and −70.62°. Circinus culminates each year at 9 p.m. on 30 July. The recommended three-letter abbreviation for the constellation, as adopted by the International Astronomical Union in 1922, is "Cir".

Circinus is a faint constellation, with only one star brighter than fourth magnitude. Alpha Circini, a white main sequence star with an apparent magnitude of 3.19, is 54 light-years away and 4° south of Alpha Centauri. Not only the brightest star in the constellation, it is also the brightest example of a rapidly oscillating Ap (RoAp) star in the night sky. It has the unusual spectral type A7 Vp SrCrEu, showing increased emissions of strontium, chromium and europium. Stars of this type have oddly localised magnetic fields and are slightly variable. Alpha Circini forms a binary star system with an orange dwarf companion of spectral type K5 and magnitude 8.5, which with a separation of 5.7 arcseconds is only discernible with a telescope. The distance between the two stars is 260 AU and they take 2600 years to rotate around a common centre of gravity. The second brightest star is Beta Circini, a white main sequence star of spectral type A3Va and a magnitude of 4.07, about 100 light-years away. It has around 1.8 times the diameter of the Sun.

Gamma Circini is a binary star 450 light-years away, whose components need a telescope of 150 mm to be seen, as they are only 0.8 arcseconds apart. The brighter component is a bluish Be star of spectral type B5IV+ and magnitude 4.51, while the dimmer component is a yellow star of magnitude 5.5. They orbit each other every 180 years. Delta Circini is also a multiple star whose components have magnitudes of 5.1 and 13.4 and orbit around a common centre of gravity every 3.9 days. The brighter component is a close eclipsing binary (specifically, a rotating ellipsoidal variable), with a minor dip of magnitude (0.1). Both are hot blue stars of spectral types O7III-V and O9.5V, respectively, and are estimated to have around 22 and 12 times the Sun's mass. Over 3600 light-years away, this system would outshine Venus at magnitude −4.8 if it were 32 light-years (10 parsecs) distant. The two main components are separated by 50 arcseconds, resolvable to the naked eye for individuals with good vision and easily discernible with a telescope.

Eta Circini is a yellow giant of spectral type G8III and magnitude 5.17, located around 276 light-years distant, and Zeta Circini is a blue-white main sequence star of spectral type B3V and magnitude 6.09, located around 1273 light-years away.

493 variable stars have been recorded in Circinus, but most have a very small range or are quite dim. Three prominent examples are Theta Circini, T Circini, and AX Circini. Theta Circini is a B-class irregular variable, ranging in magnitude from 5.0 to 5.4. T Circini has a B-type spectrum, ranging in magnitude from 10.6 to 9.3 over a period of 3.298 days, although it is actually an eclipsing binary system rather than a pulsating star. AX is a Cepheid variable that varies between magnitudes 5.6 and 6.19 over 5.3 days. It is a yellow-white supergiant of spectral type F8II+, 1600 light-years away. BP Circini is another Cepheid variable with an apparent magnitude ranging from 7.37 to 7.71 over 2.4 days. Both cepheids are spectroscopic binaries, with companions that are blue-white stars of spectral type B6 and 5 and 4.7 solar masses, respectively. BX Circini is a faint star that fluctuates between magnitudes 12.57 and 12.62 over a period of 2 hours 33 minutes. Over 99% of its composition appears to be helium. Its origin is unclear, but thought to be the result of the merger of a helium and a carbon/oxygen white dwarf.