The frequent occurrence of the term δıκαıoσυνη ("righteousness, justice") in the Sermon of the Mount (Matt 5-7) (SM) indicates one of the main themes of the text. This impression is strengthened by the fact that the term is found in strategically important locations in the composition of the SM. This article does not, however, focus on the meaning and use of δıκαıoσυνη as such. Instead, I discuss the role justice, an important component of δıκαıoσυνη, plays in the SM. To help us gain perspective on how justice was popularly conceived, a brief survey of one of the most influential ancient analyses of justice, that by Aristotle in bk. 5 of his Nicomachean Ethics, is provided. Against this background, the way the notion of justice operates in selected passages of the SM is investigated. I try to show that the text frequently manipulates and overturns conventional conceptions of justice and that this reinforces the SM's demand for a δıκαıoσυνη exceeding popular expectations.