The difference between just taxation and legal plunder is an important question for Christian ethics, dating back at least to Augustine, who raises the issue of the difference between just kingdoms and great robberies in his City of God. In this article, I provide an exposition of the thought of Thomas Aquinas on this moral question. I argue that for Aquinas, taxation is just to the extent that it constitutes fair payment to the sovereign for services rendered, not to the extent that it provides a redistributive mechanism for succoring the poor at the expense of the non-poor. In fact, there is every reason to believe that redistributive taxation would constitute robbery in the eyes of Aquinas. In defending my reading of Aquinas, I devote much space to interaction with his recent interpreter John Finnis, whose defense of redistributive taxation on Thomistic principles involves, I argue, a misunderstanding of Aquinas.