This essay engages several methodological and linguistic quandaries that have arisen in comparative modernist studies since its “transnational” or “global” turn. It does so through the work of the Spanish poet Juan Ramón Jiménez and his curious position between modernismo and English-language American (U.S.) modernism. Jiménez's Diario de un poeta reciencasado (Diary of a Newlywed Poet, 1917), written during and after his round-trip voyage between Spain and New York, is the basis for what the poet called his “rebirth” in Spanish and American poetry. In this hybrid semi-epic text, he juxtaposes translations, cross-linguistic rhymes, and citations from a host of literary histories in order to demonstrate his own shifts across modernismo, modernism, Romanticism, and symbolism. These movements cohere, he asserts, because they are all part of “modernismo.” This article uses his theoretical and critical writings from the following four decades to unpack the ways in which Jiménez's sense of modernismo is actually a forerunner — though with some important limits — of the revised, globalized, and pluralized understanding of modernism that currently prevails in Anglophone modernist scholarship. I use the term “modernism/o” to signal this shift between languages that informs his poetry and helpfully illuminates the critical and terminological problems of contemporary scholarship — some of which hinge on the limitations of translation between Spanish and English. Jiménez's wide-ranging, transhistorical genealogy of modernism/o goes back to Arabic-Andalusian poetry from medieval Spain, and he points to the Spanish-American War of 1898 as the surprising catalyst for the intersection of Spanish, Spanish American, and U.S. modernisms that he sees flourishing in his moment. From this theoretical perspective, he offers a reading of American literary history through the “great trio” of Poe, Whitman, and Dickinson, through “Midwest modernists” such as William Vaughn Moody, and through his contemporaries — Frost, Lindsay, Lowell, Millay, Eliot, and Pound — who finally cast off any notion of derivation from British models. And he makes his own work the point de repère of modernism/o's overlooked Anglo-Spanish constitution in the early twentieth century. I articulate through both his work and issues in modernist studies of the past two decades an approach to modernisms as interconnected phenomena that necessitate a more intense focus on exchange, commerce, and unsettled, constant “motion,” as demonstrated (both literally and textually) by writers who do not fit some contemporary paradigms of comparative study.