Rosen made the comments at his public swearing-in ceremony in the rotunda at San Jose City Hall on Wednesday evening.
The "Conviction Integrity Unit" will examine old cases to ensure that those convicted did in fact commit the crimes, and investigate law enforcement practices to make sure they do enough to prevent false convictions.
The cold case unit will use DNA evidence to investigate unsolved murders, rapes and violent assaults.
"Our mission is not only to prosecute the guilty because the law mandates it, but when necessary to exonerate the innocent because justice demands it," Rosen said.
The audience at the swearing-in included Rosen's wife Amber and daughters Rachel and Naomi. Also in attendance were Chief Assistant District Attorney Jay Boyarsky, San Jose Mayor Chuck Reed, council members and other city, police, and district attorney's officials.
"It sort of feels like my wedding," Rosen joked. "I feel like I married Santa Clara County."
Rosen vowed to adhere to values of service, hard work, transparency and integrity.
"A good prosecutor is one who tempers zeal with human kindness, who seeks truth and not victims, who serves the law and not factional purposes, and who approaches his task with humility," Rosen said, quoting the late Supreme Court Justice Robert Jackson.