The Triangulum Galaxy is a spiral galaxy 2.73 million light-years (ly) from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC 598. With the D₂₅ isophotal diameter of 18.74 kiloparsecs (61,100 light-years), the Triangulum Galaxy is the third-largest member of the Local Group of galaxies, behind the Andromeda Galaxy and the Milky Way.

The galaxy is the second-smallest spiral galaxy in the Local Group after the Large Magellanic Cloud, which is a Magellanic-type spiral galaxy. It is believed to be a satellite of the Andromeda Galaxy or on its rebound into the latter due to their interactions, velocities, and proximity to one another in the night sky. It also has an H II nucleus.

The galaxy gets its name from the constellation Triangulum, where it can be spotted.

It is sometimes informally referred to as the "Pinwheel Galaxy" by some astronomy references, in some computerized telescope software, and in some public outreach websites. However, the SIMBAD Astronomical Database, a professional database, collates formal designations for astronomical objects and indicates that Pinwheel Galaxy refers to Messier 101, which several amateur astronomy resources including public outreach websites identify by that name, and that is within the bounds of Ursa Major.

Under exceptionally good viewing conditions with no light pollution, the Triangulum Galaxy can be seen by some people with the fully dark-adapted naked eye; to those viewers, it is the farthest permanent entity visible without magnification, being about half again as distant as Messier 31, the Andromeda Galaxy. It is a diffuse, or extended, object rather than a starlike point, even without magnification, because of its physical extent.

Its observability without optical aid ranges from being relatively easily seen by people using direct vision in deep rural locations under a dark, clear, transparent sky, to requiring use of averted vision by observers in locations beyond the suburbs in shallow rural areas under good viewing conditions. It is one of the reference objects of the Bortle Dark-Sky Scale.

Crumey has shown that although the total apparent V-magnitude of M33 is 5.72, it has an effective visual magnitude of approximately 6.6, meaning that a precondition for visibility is that the observer can see stars at least as faint as that latter figure. This is fainter than many people are able to see, even at a very dark site.

The Triangulum Galaxy was probably discovered by the Italian astronomer Giovanni Battista Hodierna before 1654. In his work De systemate orbis cometici; deque admirandis coeli caracteribus ("About the systematics of the cometary orbit, and about the admirable objects of the sky"), he listed it as a cloud-like nebulosity or obscuration and gave the cryptic description, "near the Triangle hinc inde". This is in reference to the constellation Triangulum as a pair of triangles. The magnitude of the object matches M33, so it is most likely a reference to the Triangulum Galaxy.