For 250 years U.S. law has protected jury verdicts from being overturned due to juror misconduct or bias. A liberal Supreme Court majority has now carved out an exception for racial bias, and in an ill-defined way with no limiting principle that is likely to damage the jury system.

After a Colorado jury convicted a Mexican man of sexual harassment, two jurors signed affidavits that a retired police officer on the jury had expressed racial animus during deliberations. The juror was reported to have stated that “nine times out...