In a 4:35 p.m. tweet, he endorsed Kim Foxx, who faces off against incumbent Anita Alvarez for the Democratic nomination on Tuesday's primary election.
Alvarez has been under fire the last few months for her handling of the Laquan McDonald shooting.
He said "I'm endorsing @KimFoxxforSA because she will bring transformational change to our criminal justice system." He also linked to her campaign website.
Legend, who is the husband of model Chrissy Teigen, also posted on his verified Facebook page Sunday afternoon.
He said: "The national debate over police shootings has led policymakers and the public to begin thinking critically about the role of prosecutors in the criminal justice system. Prosecutors are critical actors in the criminal justice system holding immense power and discretion at every stage - charging decisions, bail, and sentencing that impacts defendants, victims and the larger community. It is imperative that we support and vote for prosecutors that enhance integrity and fairness throughout our criminal justice system.
"As cases from Michael Brown to Laquan McDonald put front and center the people our justice system fails everyday, we must support candidates who recognize that our system is in desperate need of transformation that puts healthy, safe communities as the core value and addresses the vast systemic racial and ethnic disparities. That's why today I'm officially endorsing Kim Foxx for Cook County State's Attorney in the March 15, 2016 Democratic primary. Kim Foxx has the experience needed to restore trust in the Cook County's State Attorney's Office and the ability to help truly redefine the role of the prosecutor in ending mass incarceration in America."