What is the highest marginal tax rate in the United States? By Investopedia | February 24, 2015 — 5:11 PM EST. A: In a marginal tax rate system, such as the one found in the United States, the more income one makes, the more one is expected to pay per dollar in income taxes. The IRS readjusts these numbers annually, so make sure that you are looking at the right income tax rate for the right year. For income made in 2015, the highest marginal income tax rate in the U.S. has been set at 39.6%. That will apply to single filers who made more than $413,201, to married joint filers who made over $464,850 and to heads of household who made at least $439,001.