Alnitak is a triple star system in the constellation of Orion. It has the designations ζ Orionis, which is Latinised to Zeta Orionis and abbreviated Zeta Ori or ζ Ori, and 50 Orionis, abbreviated 50 Ori. The system is located at a distance of several hundred parsecs from the Sun and is one of the three main stars of Orion's Belt along with Alnilam and Mintaka.

The primary star, Alnitak Aa, is a hot blue supergiant with an absolute magnitude of −6.0 and is the brightest class O star in the night sky with a visual magnitude of +2.0. It has two companions—Ab and B, the latter known for the longest time and the former discovered recently, producing a combined magnitude for the trio of +1.77. The stars are members of the Orion OB1 association and the Collinder 70 ^[de] association.

Alnitak has been known since antiquity and, as a component of Orion's Belt, has been of widespread cultural significance. It was reported to be a double star by amateur German astronomer George K. Kunowsky in 1819. Much more recently, in 1998, the bright primary was found by a team from the Lowell Observatory to have a close companion; this had been suspected from observations made with the Narrabri Stellar Intensity Interferometer in the 1970s.

The stellar parallax derived from observations by the Hipparcos satellite imply a distance around 225 parsecs, but this does not take into account distortions caused by the multiple nature of the system. Larger distances, typically closer to 400 pc, have been derived by many authors based on the orbit of the pair or the assumed properties of the components. This distance is comparable to the Orion molecular cloud complex, including the nearby Flame and Horsehead Nebulae.

Alnitak is a triple star system at the eastern end of Orion's Belt, the second-magnitude primary having a 4th-magnitude companion nearly 3 arcseconds distant, in an orbit taking over 1,500 years.

The part called Alnitak A is itself a close binary, comprising the stars Alnitak Aa and Alnitak Ab.

Alnitak Aa is a blue supergiant of spectral type O9.5Iab with an absolute magnitude of −6.0 and an apparent magnitude of 2.0. It is estimated as being up to 33 times as massive as the Sun and a diameter 20 times greater. It is some 21,000 times brighter than the Sun, with a surface brightness (luminance) some 500 times greater. It is the brightest star of class O in the night sky. In about a million years, it will expand into a red supergiant wider than the orbit of Jupiter before ending its life in a supernova explosion, likely leaving behind a black hole.

Alnitak Ab is a blue subgiant of spectral type B1IV with an absolute magnitude of −3.9 and an apparent magnitude of 4.3, discovered in 1998.