Nelson S. BeGaye made the announcement Monday in an emotional speech at the start of the Navajo Nation Council's weeklong summer session. He has served on the council since 2003.
He did not elaborate on his health condition but says he wants to spend more time with family.
BeGaye is known for championing a 2005 bill to ban uranium mining on the vast reservation that extends into Arizona, New Mexico and Utah.
Council Speaker Seth Damon says BeGaye's leadership has been soft, sturdy and compassionate.
A special election will be held to fill BeGaye’s seat.