The statue is the first of the 100 artworks donated by states for the Capitol to show a child and a disabled person, the Birmingham, Ala., News reported.
More than 400 people attended the ceremony in the Rotunda, including 35 members of Keller's family. Some of them still live near her childhood home in Tuscumbia, Ala.
Keller, the first deaf and blind person to graduate from college, realized the signs her teacher was making were words when Annie Sullivan pumped water over one hand while making the signs for w-a-t-e-r in the other.
"Helen Keller, in this statue in the Capitol, will always remind us that people must be respected for what they can do, rather than judged for what they cannot," House Speaker Nancy Pelosi, D-Calif., told the crowd.
The sculptor, Edward Hlavka, drove the statue to Washington in his pickup truck and was the only person to have seen it before its unveiling.
The statue will be on permanent display in the main hall of the Capitol visitor's center.