Sylvester Umaru Onu (born June 15, 1938) is a Nigerian judge. He has been an Associate Justice on the Supreme Court since June 1993.
Onu is from Egume, in what is now Kogi State. He attended Government College, Keffi, Ahmadu Bello University, Gibson and Weldon, and the Law School at the School of Oriental and African Studies, part of the federal University of London. He held several law-related positions before his appointment to the Court of Appeal in 1984, which he left for the Supreme Court in 1993.