Lusitanian playwrights who wrote comedias during and after the Dual Monarchy (1580–1640), when the Portuguese and Spanish thrones were united under Habsburg rule, continue to be largely unexplored. This edition highlights the contributions of one of this group’s most successful and celebrated members, Jacinto Cordeiro. It describes the sparse critical attention that Cordeiro has received as well as his life, literary career, and historical context. Most importantly, it provides a modern critical edition of Cordeiro’s most enduring play, El juramento ante Dios, y lealtad contra el amor, based on a collation of the twenty-one extant witnesses that comprise its textual tradition. Additionally, it includes an in-depth account of the transmission of the play with a stemma that documents the genealogical relationships between extant versions. It also provides an analysis of how Juramento may have been performed for seventeenth-century theatergoers, based on stage directions and performance cues written into the dialogue. In short, this edition introduces modern readers to both Jacinto Cordeiro, a bilingual author who successfully competed in a second language with the giants of Spain’s Golden Age, and El juramento ante Dios, a play whose popularity lasted two centuries.