In Roman law, ius singulare (Latin, "singular law") was a special law that applied to a certain class of persons, such as soldiers or minors, or to an individual, in contrast to ius commune civium Romanorum, the law in common to all Roman citizens.^[1] In effect, ius singulare was "a technique for incorporating new rules without having to change the old ones."^[2]

Ius singulare generally conferred an advantage to the group or individual singled out.^[3] In early Rome, privilegium was a legal enactment that might disfavor an individual or group (the privilegiarii), but in the Imperial era privilegium came to mean "privilege" and was sometimes used as a synonym of ius singulare.^[4]

As an example, military campaigning removed personnel from circumstances in the city of Rome that facilitated the procedures for drawing up a last will and testament. They were therefore exempt from the usual requirements, and wills written on the eve of battle could be held as valid.^{[citation needed]}

A ius singulare was meant to advance the public interest as a matter of utility. It required deft jurisprudence that preserved the norms of ius commune so that the exception did not become the rule,^[5] with a clear understanding that the ius singulare was contra rationem, contrary to standard legal reasoning and thus not precedent.^[6] The principle of ius singulare was one of several techniques, among them a plethora of legal fictions, that enabled Roman law to evolve flexibly without overturning its authoritative texts or challenging the fundamental traditionalism of Roman society.^[7]