The juror's statements came to light after Pena Rodriguez was convicted. Two jurors reported that a third juror colleague determined that he was guilty because Pena Rodriguez is "Mexican, and Mexican men take whatever they want."
Pena Rodriguez said the juror's views, expressed behind the closed doors of the jury room, deprived him of a fair trial.
Justice Anthony Kennedy wrote for the majority "that blatant racial prejudice is antithetical to the functioning of the jury system and must be confronted in egregious cases like this one despite the general bar of the no-impeachment rule." The court's four liberal justices joined with Kennedy to form a majority.
But the court stopped short of ordering a new trial or even laying out procedures for lower courts to follow. Instead, Kennedy said, trial courts could "consider the evidence of the juror's statement and any resulting denial of the jury trial guarantees.
Chief Justice John Roberts and Justices Samuel Alito and Clarence Thomas dissented.
"Today, with the admirable intention of providing justice for one criminal defendant, the court not only pries open the door; it rules that respecting the privacy of the jury room, as our legal system has done for centuries, violates the Constitution," Alito wrote.
The Supreme Court had resisted the call in earlier cases to examine what was said in the jury room. But several justices indicated during argument in the case in October that the allegations raised by Pena Rodriguez made for an extraordinary case.
The dispute arose after a jury convicted Pena Rodriguez of inappropriately touching teenage girls.
No other juror was alleged to have said anything improper and all 12 jurors, including the two who reported the inappropriate comments, voted to convict him.
Lawyers for Colorado and the Obama administration, which urged the court to leave jury secrecy undisturbed, acknowledged that statements attributed to the juror identified only as H.C. were indefensible. But they said there are better ways to address racial bias on juries, including closer screening of potential jurors.