In 1919, a new amir in Afghanistan named Aman Allah Khan launched an ambitious campaign to reorder his government into a constitutional monarchy. By 1923, Afghanistan had ratified its first constitution, supplemented by scores of legal and administrative codes. Whereas the latter have long been attributed to European borrowings or Kemalist imitation, this article uncovers two neglected features of Aman Allah's reformist project to argue that the making of Afghanistan's 1923 Constitution presents a distinctive path of state building in the region: Islamic legal modernism. First, by upholding the Hanafi school of Islamic jurisprudence as the basis of Afghan substantive law, Amir Aman Allah sought a cohesive national judiciary through the codification of fiqh, not European civil law. Second, by synthesizing the expertise of a diverse cast of Muslim scholars and professionals—from Afghan clerics to Ottoman and Indian technocrats recruited to Kabul—he attempted to avert a rift between “Islamic” and “secular” lawmaking.