The civil rights activist known for her crucial role in the Montgomery Bus Boycott was Rosa Parks. Her refusal to give up her bus seat to a white passenger in Montgomery, Alabama, on December 1, 1955, became a pivotal moment in the struggle against racial segregation. Parks' act of defiance sparked the boycott led by Martin Luther King Jr., mobilizing the African American community and ultimately leading to a Supreme Court ruling that declared segregation on public buses unconstitutional. Parks is celebrated as a symbol of resistance and the civil rights movement.