Sailors' superstitions are superstitions particular to sailors or mariners, and which traditionally have been common around the world. Some of these beliefs are popular superstitions, while others are better described as traditions, stories, folklore, tropes, myths, or legends. The origins of many of these superstitions are based in the inherent risks of sailing, and luck, either good or bad, as well as portents and omens that would be given associative meaning in relation to the life of a mariner, sailor, fisherman, or a crew in general. Even in the 21st century, "fishers and related fishing workers" in the U.S. have the second-most dangerous occupation, trailing only loggers.

By far the best known sailors' superstitions involve bad luck.

Sailors are taught if the sunrise is red to take warning. The day ahead will be dangerous. "Red Sky at night, Sailors delight; Red Sky in the morning, Sailors take warning." It may also be said as; "Red at morning, Sailor's warning; Red at night, Sailor's delight," or "Red sky at night, Sailor's delight; Red sky at morn, Sailor be warned."

This saying actually has some scientific validity, although it assumes storms systems will approach from the west, and is therefore generally correct only at mid-latitudes where, due to the rotation of the Earth, prevailing winds travel west to east. If the morning skies are red, it is because clear skies over the horizon to the east permit the sun to light the undersides of moisture-bearing clouds. Conversely, to see red clouds in the evening, sunlight must have a clear path from the west, so therefore the prevailing westerly wind must be bringing clear skies. Basically, this means if there is a red sky, Sun, or clouds at morning, it might mean there will be a storm, or severe winds will come. Although, if there is a red sky, Sun, or clouds at night, there will be clear skies, soft or no winds, and you have a good day ahead of you.

A "Jonah" is a long-established expression among sailors, meaning a person (either a sailor or a passenger) who is bad luck, which is based on the Biblical prophet Jonah. Clergymen are considered bad luck, as they are all of Jonah's ilk. Redheads and women are also to be avoided as passengers.

Friday is considered to be an unlucky day in some cultures, and perhaps the most enduring sailing superstition is that it is unlucky to begin a voyage or "set sail" on a Friday. However, this superstition is not universal. In the 19th century Admiral William Henry Smyth, writing in his nautical lexicon The Sailor's Word-Book, described Friday as

The Dies Infaustus, on which old seamen were desirous of not getting under weigh, as ill-omened.

(Dies Infaustus means "unlucky day".) This superstition is the root of the well-known urban legend of HMS Friday.