This Article explores an area of close parallel between legal doctrines in the contexts of Islamic law and American legal theory. In criminal law, both traditions espouse a type of “rule of lenity”—that curious common law rule that instructs judges not to impose criminal sanctions in cases of doubt. The rule is curious because criminal law is a peremptory expression of legislative will. However, the rule of lenity would seem to encourage courts to disregard one of the most fundamental principles of Islamic and American legislation and adjudication: judicial deference to legislative supremacy. In the Islamic context, such a rule would be even more curious, allowing Muslim judges to disregard a deference rule even more entrenched than the American one: a divine legislative supremacy to which judicial deference should be absolute. Yet, there is an “Islamic rule of lenity” that pervades Islamic criminal law. This Article examines the operation of and justifications for the lenity rule in the American and Islamic contexts against the backdrop of theories of law and legislative supremacy that underlie both. In both contexts, the lenity rule acts serves to expand the operation of judicial discretion. But whereas the use of American lenity is fraught and limited, Islamic lenity is relatively uncontroversial and expansive. With the Islamic rule of lenity, we see both stronger legislative supremacy doctrines and more assertions (albeit hidden) of judicial authority to legislate. An examination of the role of lenity in Islamic law with respect to American law explains differences in the scope and exercise of judicial discretion in each legal system. It can also lead us to reconsider common public law theories that characterize rules of deference to doctrines of legislative supremacy and nondelegation as a constraint on judicial discretion.