thumb|Comparison of [[bounding box of Seawaymax with some other ship sizes in isometric view.]]
A Seawaymax vessel is one of the maximum size that can fit through the canal locks of the St. Lawrence Seaway, linking the inland Great Lakes of North America with the Atlantic Ocean.
thumb|220px|right|CSL Laurentien, a Seawaymax-sized vessel
Seawaymax vessels are in length, wide, and have a draft of and a height above the waterline of .