Panamax and New Panamax are terms for the size limits for ships traveling through the Panama Canal. Formally, these limits and requirements are published by the Panama Canal Authority (ACP), titled "Vessel Requirements". These requirements also describe topics like exceptional dry seasonal limits, propulsion, communications, and detailed ship design.

The allowable size is limited by the width and length of the available lock chambers, by the depth of water in the canal, and by the height of the Bridge of the Americas since that bridge's construction. These dimensions give clear parameters for ships destined to traverse the Panama Canal and have influenced the design of cargo ships, naval vessels, and passenger ships.

Panamax specifications have been in effect since the opening of the canal in 1914. Ships that do not fall within the Panamax-sizes are called post-Panamax. In 2009 the ACP published the "New Panamax" that will be in effect when the canal's third set of locks, larger than the current two, becomes operational.