The first Hispanic U.S. Supreme Court justice has been on the bench for nearly five years but had never written an opinion addressing race in America until today.
In strong terms that referred to the slights minorities face because of skin color, Sotomayor declared the court's majority was ignoring a U.S. history of discrimination and the needs of those on the margins of society.
For the first time since her 2009 appointment, she also took the unusual step of reading portions of the opinion from the bench, which dissenting justices sometimes do to draw special attention to their views.
Speaking for 12 minutes, nearly as long as Justice Anthony Kennedy spoke for the court's majority, Sotomayor said her colleagues "fundamentally misunderstand" the issue of racial bias at the heart of the dispute and chided them for not talking more openly about it.
Sotomayor's actions answered a question that has lingered since 2009 when Barack Obama, the first black U.S. president, nominated her to the court: how would she draw on her personal experience and possibly differentiate herself from previous justices?
In her 2013 memoir Sotomayor wrote of benefiting in the 1970s from affirmative action at Princeton University and Yale Law School. On Tuesday, her words flashed across Twitter and other social media.
Still, her dissent is unlikely to change the legal landscape. Based on the court majority's stance Tuesday and the conservative majority's skepticism about remedies in racial cases dating back to 2007, the court is likely to continue chipping away at affirmative action.
Sotomayor, whose opinion was joined only by senior liberal Justice Ruth Bader Ginsburg, is part of a minority on the left, and her side is not likely to gain more votes in the near future. The justice most likely to retire is the 81-year-old Ginsburg.
Since Sotomayor joined the nine-justice court, it has taken up a handful of major cases involving voting rights, affirmative action and immigration. In those disputes, Sotomayor signed the opinions of more senior justices, whether in dissent or the majority.
In last term's challenge to federal voting rights law, for example, Sotomayor joined Ginsburg's dissent claiming that the five-justice majority had rolled back needed protections for minority voters in places with a history of discrimination at the polls.
In a case last term involving admissions at the University of Texas, Sotomayor signed Kennedy's opinion upholding racial criteria in admissions and asking a lower court to take another look at the Texas program.
In her dissent on Tuesday, Sotomayor said the Michigan ban was unfair. She said the voter-approved ban allowed college administrators to have admissions policies favoring, for example, students whose relatives are alumni but blocks race-based criteria. The law, she wrote, "restructures the political process in Michiganin a manner that places unique burdens on racial minorities" to win changes in admissions policies.
Her opinion addressed broader racial issues, too, as she noted the stigma of skin color and said the court majority should recognize how much race matters.
"The way to stop discrimination on the basis of race is to speak openly and candidly on the subject of race," she wrote.