Throughout the history of Icelandic, invariant particles, which do not inflect for semantic or syntactic features of the antecedent, are the typical markers of relative clauses (Þráinsson 2007). Another, putatively foreign strategy—relativization with interrogative–relative pronouns—is archaic in Modern Icelandic, but is frequent between the sixteenth and eighteenth centuries, a period featuring a high degree of literary translation from languages which themselves possessed interrogative–relativization strategies. In this paper, I examine both internal processes of grammaticalization and the effects of contact on the development of this strategy from Old‐to‐Early Modern Icelandic by comparing the translation of the morphosyntatic structures of relative clauses among the languages involved in this contact scenario: Icelandic, German, and Latin. Using the Icelandic Parsed Historical Corpus (Wallenberg et al. 2011), I conduct a diachronic corpus study that quantifies the development of this relativization strategy. I argue that the proliferation of the Icelandic interrogative–relativization strategy is a contact phenomenon stimulated by pattern borrowing from (primarily) Latin, but contextualized and made possible by a state of systemic variability and instability in the relative paradigm in Early Modern Icelandic.