The 15th Amendment to the US Constitution was ratified on February 3, 1870 during Reconstruction. Along with the 13th amendment and the 14th amendment, it is one of the three Reconstruction amendments.
Text of the 15th Amendment
The right of citizens of the United States to vote shall not be denied or abridged by the United States or by any State on account of race, color, or previous condition of servitude.
The Congress shall have power to enforce this article by appropriate legislation.