The motion to elect Kumar was also supported by members of the opposition Bharatiya Janata Party (BJP).
Neerja Choudhury, a political analyst based in India, said Kumar's nomination was a "conscious decision" on the part of Congress to strengthen its support base among India's 160 million Dalits, also known as "untouchables".
Shunned by higher castes, Dalits generally perform the lowliest occupations, including scavenging on rubbish dumps, and are the poorest in terms of income, literacy and land.
As the daughter of Jagjivan Ram, a prominent Dalit leader and former deputy prime minister, Kumar replaces Somnath Catterjee, a communist lawmakrer who held the post for the previous five years.
Manmohan Singh, India's prime minister, declared her appointment as an "historic occasion".
"For the first time a woman member has been elected speaker and that too a woman from the Dalit community.
"In electing you ... we members of parliament pay tribute to the women of our country and the great contribution that they have made," he said.