By now it is a commonplace to describe early Christian martyr discourse as rhetorical constructions rather than disinterested historical accounts. Yet rarely have scholars engaged this construction within the technical frame of late antique rhetorical theory and practice, and when they have, they do so mainly with an eye to stylistic concerns rather than the argumentative heart of late antique rhetoric: forensic stasis theory. This short communication begins to fill this lacuna by arguing that Augustine’s rejection of Donatist martyr claims ought to be read in the context of forensic rhetoric, particularly as a quaestio or controversia of (de)finitio. In particular, Augustine’s repeated argument that non poena sed causa (not the punishment, but the cause) makes a martyr represents a dispute over the defining proprium of martyrdom. This paper’s argument develops in two stages. First, I analyze the background to forensic rhetorical theory and issues of definition in Cicero (esp. de Oratore 2.107-110) and Quintilian. I next show how Augustine’s anti-Donatist martyr rhetoric fits into this schema by looking at en. Ps. 32, where he pleads his case to his congregation, and ep. 185, where he presents the same argument to the tribune Boniface. In both cases, Augustine incorporates Matthew 5:10 to supply the defining proprium of “righteousness” as the necessary causa for true martyrdom. Thus, in constructing his anti-Donatist definition of martyrdom, Augustine deploys classical forensic rhetorical practice both in his argumentation and in his reading of scripture.