The Clarke–McNary Act of 1924 (ch. 348, 43 Stat. 653, enacted June 7, 1924) was one of several pieces of United States federal legislation which expanded the Weeks Act of 1911. It was named for Representative John D. Clarke and Senator Charles McNary.
The Weeks Act had allowed the purchase of land to enlarge the National Forest System. Two years after the Weeks Act had been passed, over 700,000 acres (2,800 km²) had been purchased for the National Forest system in the Eastern United States. More than 2 million acres (8,000 km²) of land had been purchased by 1920. The Clarke–McNary Act made it easier for the Forest Service to buy this land. It made it much easier for them to buy land from willing sellers within predetermined national forest boundaries. The Clarke–McNary Act enabled the Secretary of Agriculture to work cooperatively with State officials for better forest protection, chiefly in fire control and water resources. It also provided for continuous production of timber. Additionally, under the Clarke–McNary Act, the United States Department of Agriculture (USDA) began working with private forestland owners in reforestation. This was done by broadening the cooperative efforts to include producing and distributing tree seedlings and providing forestry assistance to farmers. These laws also gave a strong impetus to States to establish and support State forestry agencies. All 50 states now have a state forestry agency or forestry extension agency.