The dialectical-theological origins of the politically- and ethically-charged concept of alterity are well-known within the philosophy of religion. Intellectual histories of this concept tie it too exclusively to the notion of distance or διάστασις in Karl Barth’s early Römerbrief, however, and so miss Barth’s Trinitarian reinterpretation of God’s otherness in his later work. Taking as my hermeneutical key a cipher, the ‘sign of Jonah,’ that emerges in Church Dogmatics IV/1, I show that Barth’s mature doctrines of temporality and filiation understand alterity as a moment of divine life. Jesus’ agony in the garden of Gethsemane marks the climax of Barth’s self-reinterpretation: world history inheres within the Christological situation of paternal abandonment. The political-theological conclusions Barth draws from the ‘sign of Jonah’ dovetail with alterity discourses’ antitotalitarian aspirations but suggest that these aspirations’ structural coherence rest on the magisterial Reformers’ Christological and ecclesiological commitments.