Class 40 is a class of monohull sailboat and a yacht primarily used for short handed offshore and coastal racing. The class is administered by International Class 40 Association which is recognised by the World Sailing.

The "Class 40" can be designed by anyone provided the boat fits within the measurement rule defined. This rule takes the form of a box rule.

The association is responsible for defining and updating the class rules for the boats, and has the additional aims of co-ordinating events.

Class 40 monohulls outnumbered all other boat classes during their first appearance in a world-class competition, the 2006 Route du Rhum, with 25 boats registered.

The Class 40 monohulls are high-performance racing boats, designed principally for single-handed or small crew offshore competition.