Well known as a mystic, Catherine of Siena has been credited with pope Gregory XI’s return to Rome from Avignon, with convincing him to pursue a crusade and with playing a major role in making peace between the Papal League and Italian City states. This narrative ascribes these accomplishments to Catherine’s extraordinary gifts, a fruit of her mystical experience. Contemporary historical research, however, shows that Catherine was chosen by ecclesiastical authorities to advocate for papal policies. She was guided to causes (...) and policies she should pursue, which were already determined by the pope and his advisors. In light of this historical research and from the perspective of Christian spirituality studies, I examined Catherine’s letters to pope Gregory XI to analyse and interpret her spirituality of political engagement. Her multiple authoritative exhortations to Gregory XI telling him how her wisdom applied to the choices before him can be read as a spirituality of papal governance and as ‘formation’ for his critical leadership of the church. By means of this article I have shown that Catherine’s advice was intended for the salvation and transformation of others and for the good of the church so that it might best mediate salvation. This goal was her interpretation of the mission entrusted to her through her spiritual advisor. Her own spirituality in daring to forcefully ‘form’ the pope and in espousing papal causes by preaching her creative integration of Gospel wisdom was witness to an unselfconscious courage and utter trust in God acting through her; a fruit of her own profound journey of transformation.Contribution: This article offers a new interpretation of Catherine of Siena’s engagement in papal politics and related spirituality. Catherine was chosen for this task by ecclesiastical authorities, then construed this mission as a call to ‘form’ the pope; thus, her letters to Gregory XI are interpreted as a spirituality of papal governance. (shrink)