Emerging critiques of mainstream accounts of secularism reveal the imbrication of the sacred and the secular in ‘secular’ states. In the context of colonial Northern Nigeria, this sacred-secular entanglement, which took the form of the co-option of Islam for the colonial ‘secular’ enterprise, did not leave Islam unchanged. Co-opting Islam for the colonial project necessitated the making of an Islamic Law amenable to the colonial state. With a focus on criminal law, this article narrates the making of a British Colonial Islamic law in Northern Nigeria through the unprecedented expansion of siyasa. Departing from orthodox accounts of Islamic law's reification in colonial Northern Nigeria and heterodox assertions of its erosion by the colonial state, this article argues that neither the reification nor the erosion accounts illuminates the relationship between the colonial state and Islamic law. To show how the colonial state could assert secularism while co-opting Islam, this article presents a narrative of reform that foregrounds the following questions: Who had (and exercised) the power to decide what Islamic law was? How was the exercise of this power justified? How did the exercise of this power fit with the broader colonial project of governing religious difference? What were the consequences of these processes for Islamic law, institutions and colonial subjects?