Forecastle also spelled fo'c's'le (pronounced ) originally meant the upper deck of a sailing ship, forward of the foremast. The forward part of a ship with the sailors' living quarters is also called forecastle. Related to the latter meaning is the phrase "before the mast" which denotes anything related to ordinary sailors (as opposed to a ship's officers).

The term "forecastle" relates to Medieval shipbuilding, where ships of war were usually equipped with a tall, multi-deck castle-like structure in the bows of the ship which served as a platform for archers to fire down on enemy ships and could also be used as a defensive stronghold if the ship was boarded. A similar but usually much larger structure was at the aft end of the ship, often stretching all the way from the mainmast to the stern. Having such tall upperworks on the ship was of course detrimental to sailing performance. As cannon were introduced and gunfire replaced boarding as the primary means of naval combat during the 16th century, the medieval forecastle was no longer needed, and later ships such as the galleon had only a low, one-deck high forecastle. Some sailing ships had no forecastle as such at all but the name was still being used to indicate the foremost part of the upper deck.