Catholicism and Elizabethan Seafaring is the first study to focus on Elizabethan Catholic seafarers who participated in the Anglo-Spanish conflicts. It demonstrates that Catholic identities persisted among English sailors operating between England and Spain, challenging longstanding assumptions that early modern English Catholics were predominantly land-bound and conservative, in contrast to their supposedly more progressive Protestant counterparts. The study traces shifts in religious culture aboard English ships in the 16th century and highlights the significance of English Catholic identities in both England and Spain. It examines how these mariners behaved and adapted in both countries, and how their religious and national identities, as well as their loyalties, interplayed, changed, and evolved, particularly under the conditions of Anglo-Spanish confrontation and the broader era of confessionalisation. The focus also includes the motivations that led Catholics to serve at sea on Protestant or Catholic ships, their religious practices, interactions with wider society– including radical Protestants–and the challenges they faced in both contexts. Through meticulous archival research, the monograph uncovers the Catholic identity aboard Elizabethan ships, often concealed by its carriers, and reconstructs the previously forgotten biographies of Englishmen serving on Habsburg vessels. It brings together cases drawn from both Spanish and British archives.