“Spanish American modernismo,” wrote Octavio Paz in 1972, “has no connection to what in English is called ‘modernism.’” Indeed, for a long time there was consensus in both critical traditions that “despite some parallels,” as Astradur Eysteinsson put it, “the differences between the two concepts are too many to warrant their critical coalescence.” In recent years, however, it has become the rule to discuss Latin American and Spanish modernismos within the Anglo-Germanic notion of modernism, as part of the broader concept of “global modernisms.” But how did two of the most important aesthetic concepts of the twentieth century from two distinct traditions go from misleading cognates to variants of the same phenomenon? This article offers a comparative history to explain the conditions of their mutual (un)translatability. It presents their divergent beginnings, briefly surveys their independent developments, and finally argues that, in the past few decades, both have similarly turned from differential to relational concepts mainly by transforming their relationship to mass culture. They have thus gone from high-literary ideologies of exclusion to fields of research on the networks and dynamics of modern culture. As they did, however, their standing within their respective critical traditions has changed in opposite ways.