Abstract: Racial and gender disparities are prevalent in the criminal justice system, but the sources of these disparities remain largely unknown. This paper investigates whether judge political affiliation contributes to these disparities using data on over 500,000 federal defendants linked to sentencing judge. Exploiting random case assignment, we find that Republican appointed judges sentence black defendants to 3.0 more months than similar non-blacks and female defendants to 2.1 fewer months than similar males, compared to Democratic appointed judges. Disparities by judge political affiliation cannot be explained by other judge characteristics and grow substantially larger when judges are granted more discretion.