The Complete America's Great Trails Act creates a new federal tax credit for landowners who donate or restrict the use of their property to protect National Scenic Trails and the surrounding corridor. Specifically, taxpayers can claim an income tax credit equal to the fair market value of their conservation contribution to a National Scenic Trail, which includes the trail itself and land up to 2,640 feet wide on either side of it, with some flexibility for existing structures and varied terrain. The credit can be carried forward for up to ten years if it exceeds the taxpayer's tax liability in a given year, though it cannot be combined with a charitable deduction for the same property. The bill requires the Secretary of the Interior to study the credit's effectiveness within four years and evaluate whether it should be made refundable or transferable, and the tax provisions take effect upon enactment.