The prominent figure who delivered the powerful speech titled "Injustice anywhere is a threat to justice everywhere" was Martin Luther King Jr.. This quote is from his famous "Letter from Birmingham Jail," written in 1963 during his campaign against racial segregation and oppression in Birmingham, Alabama. King's words emphasized the interconnectedness of social justice, advocating for a nonviolent approach to achieving equality. His eloquence and moral clarity helped galvanize the Civil Rights Movement, inspiring generations to continue the fight for justice and equality in America and beyond.