Horologium (Latin hōrologium, the pendulum clock, from Greek ὡρολόγιον, lit. 'an instrument for telling the hour') is a constellation of six stars faintly visible in the southern celestial hemisphere. It was first described by the French astronomer Nicolas-Louis de Lacaille in 1756 and visualized by him as a clock with a pendulum and a second hand. In 1922 the constellation was redefined by the International Astronomical Union (IAU) as a region of the celestial sphere containing Lacaille's stars, and has since been an IAU designated constellation. Horologium's associated region is wholly visible to observers south of 23°N.

The constellation's brightest star—and the only one brighter than an apparent magnitude of 4—is Alpha Horologii (at 3.85), an aging orange giant star that has swollen to around 11 times the diameter of the Sun. The long-period variable-brightness star, R Horologii (4.7 to 14.3), has one of the largest variations in brightness among all stars in the night sky visible to the unaided eye. Four star systems in the constellation are known to have exoplanets; at least one—Gliese 1061—contains an exoplanet in its habitable zone.

The French astronomer Nicolas-Louis de Lacaille first described the constellation as l'Horloge à pendule & à secondes (Clock with pendulum and seconds hand) in 1756, after he had observed and catalogued almost 10,000 southern stars during a two-year stay at the Cape of Good Hope. He devised fourteen new constellations in previously uncharted regions of the southern celestial hemisphere, which were not visible from Europe. All but one honoured scientific instruments, and so symbolised the Age of Enlightenment. The constellation name was Latinised to Horologium in a catalogue and updated chart published posthumously in 1763. The Latin term is ultimately derived from the Ancient Greek ὡρολόγιον, for an instrument for telling the hour.

Covering a total of 248.9 square degrees or 0.603% of the sky, Horologium ranks 58th in area out of the 88 modern constellations. Its position in the southern celestial hemisphere means the whole constellation is visible to observers south of 23°N. Horologium is bordered by five constellations: Eridanus (the Po river or Nile river), Caelum (the chisel), Reticulum (the reticle), Dorado (the dolphin/swordfish), and Hydrus (the male water snake). The three letter abbreviation for the constellation, as adopted by the International Astronomical Union in 1922, is "Hor". The official constellation boundaries are defined by a twenty-two-sided polygon (illustrated in infobox). In the equatorial coordinate system, the right ascension coordinates of these borders lie between 02^h 12.8^m and 04^h 20.3^m , while the declination coordinates are between −39.64° and −67.04°.

Horologium has one star brighter than apparent magnitude 4, and 41 stars brighter than or equal to magnitude 6.5. Lacaille charted and designated 11 stars in the constellation, giving them the Bayer designations Alpha (α Hor) through Lambda Horologii (λ Hor) in 1756. In the mid-19th century, English astronomer Francis Baily removed the designations of two—Epsilon and Theta Horologii—as he held they were too faint to warrant naming. He was unable to find a star that corresponded to the coordinates of Lacaille's Beta Horologii. Determining that the coordinates were wrong, he assigned the designation to another star. Kappa Horologii, too, was unable to be verified—although it most likely was the star HD 18292—and the name fell out of use. In 1879, American astronomer Benjamin Apthorp Gould assigned designations to what became Mu and Nu Horologii as he felt they were bright enough to warrant them.

At magnitude 3.9, Alpha Horologii is the brightest star in the constellation, located 115 (± 0.5) light-years from Earth. German astronomer Johann Elert Bode depicted it as the pendulum of the clock, while Lacaille made it one of the weights. It is an orange giant star of spectral type K2III that has swollen to around 11 times the diameter of the Sun, having spent much of its life as a white main-sequence star. At an estimated 1.55 times the mass of the Sun, it is radiating 38 times the Sun's luminosity from its photosphere at an effective (surface) temperature of 5,028 K.

At magnitude 4.93, Delta Horologii is the second-brightest star in the constellation, and forms a wide optical double with Alpha. Delta itself is a true binary system composed of a white main sequence star of spectral type A5V that is 1.41 times as massive as the Sun with a magnitude of 5.15 and its fainter companion of magnitude 7.29. The system is located 179 (± 4) light-years from the earth.

At magnitude 5.0, Beta Horologii is a white giant 63 times as luminous as the Sun with an effective temperature of 8,303 K. It is 312 (± 4) light-years from Earth, and has been little-studied. Lambda Horologii is an ageing yellow-white giant star of spectral type F2III that spins around at 140 km/second, and is hence mildly flattened at its poles (oblate). It is 161 (± 1) light-years from Earth.