Oswald von Nell-Breuning SJ (March 8, 1890 – August 21, 1991) was a Roman Catholic theologian and sociologist.
Born in Trier, Germany into an aristocratic family, Nell-Breuning was ordained in 1921 and appointed Professor of Ethics at the University of Frankfurt am Main in 1928. He was instrumental in the drafting of Pope Pius XI's social encyclical Quadragesimo anno (1931), which – like the earlier Rerum novarum (1891), after which it was named – dealt with the "Social Question" and developed the principle of subsidiarity. Nell-Breuning was not allowed to publish from 1936 to the end of Nazi Germany in 1945. He died in Frankfurt am Main.