The Naval Act of 1938, known as the Second Vinson Act, was United States legislation enacted on May 17, 1938, that "mandated a 20% increase in strength of the United States Navy",[1] allocating $1.09 billion (equivalent to $18.5 billion in 2023 relative to GDP inflation[2]) for it.

[3] It represented the United States' response to the Japanese invasion of China, the German annexation of Austria[4] and the disintegration of the naval treaty system established in 1922 when both Japan and Italy refused to sign the Second London Naval Treaty of 1936.

The act was sponsored by Carl Vinson, a Democratic Congressman from Georgia who was Chairman of the House Naval Affairs Committee.

Every other category of ships was already mandated to be built up over time to the maximum allowed under-age tonnage by the first Vinson Act of 1934.

Older vessels could be retained in active service longer (a vessel that was over-age and allowed to be replaced had to be scrapped if it was in fact replaced) and the size to which to build up to eventually was now increased for destroyers, submarines and cruisers (cruisers could not have been commissioned before 1941 though without the 1938 act beyond the point of what tonnage would have been gained from legally scrapping CL-4 and CL-5).