“Reason of state” is a concept that is rarely used in contemporary legal and political philosophy, compared to everyday parlance; “public reason,” in contrast, is ubiquitous, especially in liberal philosophy, as a legitimacy-conferring device. In this article it is argued that the unpopularity of the notion of “reason of state” is partly due to its notorious ambiguity. Three different usages of the notion can be identified: a “thin” usage (where “reason of state” is equivalent to the common good); an “ironical” usage (where it is used pejoratively to denounce it as a pretext for application of illegitimate or illegal means); and a “pre-emptive” usage (where “reason of state” functions as a legitimate second-order exclusionary reason used to override otherwise mandatory first-order rules of action). It is argued that only the “thin” usage is helpful in a by-and-large liberal-democratic context. The article then discusses the main dilemmas related to the concept of public reason, especially in its most influential, Rawlsian interpretation, and defends the concept against common critiques. Finally, the two concepts of “reason of state” and public reason are compared, and it is argued that a “thin” usage of “reason of state” is functionally equivalent to public reason, and that both resonate with the theory of “input democracy” (focusing, as it does, on the legitimacy of reasons—or motivations—for applying coercive rules to individuals). The article also identifies a problematic feature of “reason of state”: its emphasis on the state as a privileged interpreter of such reasons and/or as identifying the pool of actors within which the “constituency” of public reason is ascertained. There are good reasons to resist both of these consequences: the former because of its potentially authoritarian consequences, the latter because of reasons provided by cosmopolitan political conceptions.