The load line, also known as Plimsoll line, indicates the legal limit to which a ship may be loaded for specific water types and temperatures in order to safely maintain buoyancy, particularly with regard to the hazard of waves. The load line is a waterline that corresponds to the maximum draft of the ship, thus yet another name, load waterline. Varying water temperatures will affect a ship's draft, because warm water is less dense than cold water, providing less buoyancy. In the same way, fresh water is less dense than salinated or seawater, with a similar lessening effect upon buoyancy. The rules for international load lines are defined by the International Convention on Load Lines from 1966. For inland water transport regional, national or local rules apply.

Load lines are indicated by special markings on the hull. The marking for the main load line, the summer load line, is called load line mark or Plimsoll mark (positioned amidships), the marks for other conditions are named after the condition suffixed with "load line" (e.g. winter load line).

The purpose of a load line is to ensure that a ship has sufficient freeboard (the height from the waterline to the main deck) and thus sufficient reserve buoyancy. The freeboard of commercial vessels is measured between the lowest point of the uppermost continuous deck at side and the waterline and this must not be less than the freeboard marked on the load line certificate issued to that ship. All commercial ships, other than in exceptional circumstances, have a load line symbol painted amidships on each side of the ship. This symbol is also permanently marked, so that if the paint wears off it remains visible. The load line makes it easy for anyone to determine if a ship has been overloaded. The exact location of the load line is calculated and verified by a classification society and that society issues the relevant certificates. This marking was invented in 1876 by Samuel Plimsoll.

In the Middle Ages, the Venetian Republic, the Republic of Genoa and the Hanseatic League required ships to show a load line. In the case of Venice this was a cross marked on the side of the ship, and of Genoa three horizontal lines.

The first 19th-century loading recommendations were introduced by Lloyd's Register in 1835, following discussions among shipowners, shippers and underwriters. Lloyd's recommended freeboards as a function of the depth of the hold (three inches per foot of depth, 250 mm/m). These recommendations, used extensively until 1880, became known as "Lloyd's Rule".

The creators of the 'rule' did not enjoy the prominence it gained, knowing it was an oversimplification that did not take into account depth, sheer, and other form factors relating to the size and shape of the hull.

In 1869, after increased loss of ships due to overloading, a bill was introduced to Parliament that was amended to include the provision that the draught of water when a vessel was leaving port should be recorded. The bill expired, failing to be passed before the end of the parliamentary session..

The law was re-committed the following year and passed as the Merchant Shipping Act 1871, requiring a scale in feet on the stem and stern of a ship in six-inch high letters to coincide with the draught line.