This artcle considered the nautical title of Captain. Please see "Captain" for information concerning the rank

Captain is the traditional customary title given to the person in charge of a ship at sea, but on most legal documents in the merchant shipping industry, he or she is correctly referred to as the ship's Master. A nautical "captain" may be a civilian or a naval commissioned officer of any rank.

The Master of a merchant ship will be qualified as a Master Mariner. As the commander of a vessel under way, a nautical captain has enormous legal powers, including the right to use deadly force to suppress piracy and mutiny. Contrary to popular belief, the captain of a ship does not have any special legal power to conduct a marriage. A captain does, however, have the legal right to administer morphine to a patient, a legal right shared only with doctors and pharmacists.

The captain of a ship at sea is in absolute command of that vessel even if higher-ranking persons are aboard. If higher-ranking persons give orders to the nautical captain, such persons are very careful to say what they want done rather than specifying how the orders are to be carried out, because even higher rank does not give them the right to interfere in how a captain runs the ship.

On shore, a harbormaster, as the equivalent "chief" of a port, was sometimes titled "captain" if he had merchant marine or naval rank and professional service in command at sea, but this is rare today.

The traditional sleeve emblem for captains is four gold stripes (often called "rings") on the lower sleeve or shoulderboard. Many navies follow the precedent of the Royal Navy and have an "executive loop" on the top or inner ring. In the Royal Navy, the stripes and rings are called Nelsons if the ring sits above the stripe, and Half Nelson if only half above the stripe. Oftentimes harbormasters have a foul anchor or other local symbol on the gold rings.

The officer who is ranked immediately below the captain of a ship is designated the chief mate, or first officer (also executive officer on Naval vessels), and is responsible for implementing the orders of the captain as well as conferring with the captain on matters concerning the ship. This "second in command" is typically responsible (along with the senior enlisted petty officer) for maintaining minor discipline on the ship. Ranked below the chief mate are the second mate (navigational officer) and the third mate (safety officer).

In older times, a captain was a nobleman given responsibility over a ship, but was not likely to have any nautical experience. The next officer of the ship would be the ship's master, who would carry out the executive functions of a captain, while the titular captain filled a ceremonial and legal role.