A lateral buoy, lateral post or lateral mark, as defined by the International Association of Marine Aids to Navigation and Lighthouse Authorities, is a sea mark used in maritime pilotage to indicate the edge of a channel.

Each mark indicates the edge of the safe water channel in terms of port (left-hand) or starboard (right-hand). These directions are relative to the direction of buoyage; this is usually a nominally upstream direction. In a river, the direction of buoyage is towards the river's source; in a harbour, the direction of buoyage is into the harbour from the sea. Where there may be doubt, it will be labelled on the appropriate chart. Often the cardinal mark system is used instead when confusion about the direction would be common.

A vessel heading in the direction of buoyage (e.g. into a harbour) and wishing to keep in the main channel should:

Marks are distinguished by their shape and colour, being red or green.

For historical reasons, two different schemes are in use worldwide, differing in their use of colour. Previously there had been 30 different buoyage systems, before IALA rationalised the system. In 1980 on a conference convened by IALA, they agreed to adopt the rules of a new combined system, which combined the previous two systems (A and B) into one system, with two regions (A and B).

The IALA defines them as Region A and Region B:

Port hand marks are cylindrical (also known as a "can"), pillar or spar in shape. If a top mark is attached it has a cylinder (can) shape.

Starboard hand marks are conical, pillar or spar in shape. If a top mark is attached it has a conical shape with the point facing upwards.