船长（英文：Captain、Master或者Ship Master），是船上拥有航行执照中最高端的航海指挥官。船长的职责在于维护全船的安全及有效的运作，包括乘载货物的管理维护、航行、船员管理以及确保船舶符合港口国及国际公约之规定，和船籍国与船东(航商)的政策。武装船只（如军舰及海巡船舰）的船长通常称为「舰长」。

船长负有在船舶发生事故或是旅客、船员发生事故(生病、受伤或死亡)时，提出书面报告及提供有关案件的说明与证明的责任及对其负责。

不同于车辆上的司机及飞机上的机师，中大型船舶的船长通常只会作出指挥而不会直接操控船只，实际操控工作由舵工及船副负责，商船船长仅在靠离码头于驾驶台甲板操纵船舶以维持所需要的操作精准度，并且避免传令指挥的误差造成船舶损害。如果船舶设有三副，则船长不执行站更（stand watch）。

A Sea Captain (also called a Captain or a Ship's Captain or a Master or a Shipmaster) is an high grade licensed mariner in ultimate command of the vessel. The Captain is responsible for its safe and efficient operation, including: ship's seaworthiness, ship's safety and security conditions, cargo operations, navigation, crew management and ensuring that the vessel complies with local and international laws, as well as company and flag state policies. All persons on board, particularly during navigation, including, public authorities, state authorities, officers and crew, other shipboard staff members, passengers, guests and pilots, are under the captain's authority and are his or her ultimate responsibility.