Blackfoot verbs are marked with nominal agreement morphology in relative clauses, in place of verbal inflection. These relative clauses have previously been analyzed as nominalizations. The present study shows that a nominalization analysis makes incorrect predictions about the morphological composition of relative clauses, as well as the availability of non-agentive and possessive constructions, and adjectival modification. This study demonstrates that relative clauses can relativize subjects, direct objects, and indirect objects. Based on observations about obviation, recursion, long-distance extraction, inflection, and word order, I propose that relative clauses are participles: clausal entities with a nominal superstructure. This accounts for their mixed clausal and nominal properties, and provides an analysis of Blackfoot relative clauses similar to those proposed for related Algonquian languages.